How should one deal with people who cheat or have bad sportsmanship? There is a simple answer which would make this article very short: call for an umpire. However, umpires are not always available (and most of your matches will likely be practice matches anyway, where there are normally no umpires), so sometimes you'll have to deal with this on your own, especially if it's only poor sportsmanship, not outright cheating. Besides, you don't want to call for an umpire every time you think an opponent looks at you funny, so when possible, deal with the problem on your own.
Cheaters cheat because they want to win. There are limits to how you can deal with this short of calling for an umpire. If the opponent simply calls the score wrong, the remedy might be to simply call the score out loud every point, so the score is absolutely clear to you, your opponent, and anyone watching. Cheaters don't like this because it's hard to argue about the score when it's been called out loudly and clearly every point.
But there are other types of cheaters. They may serve illegally; use illegal surfaces; call lets on points that are not lets; claim their shot hit the edge or that your edge ball missed; and many other ways. You might be able to deal with some of these on your own. For example, if an opponent serves illegally (probably the most common form of "cheating"), politely explain why the serve wasn't legal. Ideally, catch the illegal serve and explain why the serve was illegal. If you attempt to return it, then the point counts. But you can still ask the opponent to serve legal between points.
If an opponent refuses to serve legally, and you can't get an umpire, then you are basically stuck, so deal with it. In tournaments you usually can get an umpire for this, but rarely in practice. And surprisingly, many umpires are reluctant to call many illegal serves. So you may have to learn to deal with some illegal serves. (Some illegal serves don't really give much of an advantage to the server, other than the fact that if they have to serve legally, they wouldn't be able to use their normal serves and they'd have difficulty serving effectively. Other illegal serves give a direct advantage to the serve, such as hiding contact, throwing the ball backwards into the racket, or excessively short tosses.)
Illegal surfaces are usually easier to deal with. You are allowed to examine an opponent's racket at the start of the match. If he has an illegal surface, even if there is no umpire you can ask the referee to look at it, and let him handle it. If there is no umpire or referee (i.e. usually a practice or perhaps a league match), then you'll have to deal with it on your own, and ask him to use a legal surface, or (as often happens in practice matches), just deal with it, and decide later whether to avoid playing that person again until he gets a legal surface. Except possibly in practice, you'll rarely have to deal with a player using the same color on both sides, but this can come up. Probably the most common illegal surface is frictionless long pips, which usually comes about when an opponent takes a legal long pips and treats it (usually with heat) to make them frictionless. Unfortunately, this is difficult for umpires or referees to judge.
Other types of cheating are harder to deal with. There's not a whole lot you can do if your opponent claims your edge ball missed or hit the side, or that his shot off the end or into the side hit the edge, or if he calls illegal lets or disagrees with your own rightly-called let. Of course, he may have just not seen what you saw, or perhaps you missed seeing what he saw, so don't be too quick to judge the opponent a cheater. Regardless of who is right, the only way to resolve a dispute like this is to call a let, which favors the one who is either wrong or outright cheating. It's up to you whether to call for an umpire (if available), using your own judgment over whether you think it will happen again.
Now the good news about cheaters: there are surprisingly few of them. Part of this is that repeat offenders get to be known, since not only opponents but people on the sidelines see it happening, and so most who might cheat quickly stop rather than face ridicule. However, there are always a few players, even at tournaments, who are regular cheaters. They are often well known to referees, who often watch these players and are quick to assign an umpire if needed. I've played tournaments where well-known cheaters informally had full-time umpires assigned to their matches.
There's a large overlap between cheating and poor sportsmanship, since cheating is simply a major form of poor sportsmanship. Players with poor sportsmanship outnumber actual cheaters. At tournaments there simply are not enough umpires to deal with every problem, so unless an opponent actually cheats, try to deal with most poor sportsmanship on your own.
There are two types of people who have bad sportsmanship. There are those where poor sportsmanship is simply a bad habit. And there are those who do it intentionally to gain an edge.
Examples of poor sportsmanship are forcing you to wait for them as they show up late; taking excessive time between points; constant complaining; any type of derogatory or belittling talk at an opponent; excessive yelling; or general bad behavior. Sports can bring out both the best and worst in people, and in many cases, it's the worst. One way to deal with most of this, if it doesn't actually break the rules, is to ignore it. Better still, become stronger because of it. If an opponent is constantly complaining or yelling, then he's obviously under great emotional stress. What a huge advantage that is for you if you are calm and relaxed!
The worst type of poor sportsmanship is when it's done intentionally to gain an edge. Suppose you show up for a match, and your opponent intentionally makes you wait for him. You can't really prove he did it on purpose, but the effect is the same: you are stuck out on the court waiting for his grand entrance. Just smile to yourself, knowing the opponent is so worried about the match he feels he needs even this tiny edge - and by doing so, you gain the edge.
The same is true of other types of bad behavior. Some opponents yell a lot between points; ignore it, or perhaps (if it is in your nature) occasionally yell yourself when you win a big point. The key here is that you don't want to feel intimidated by the opponent's yelling. Again, remember he's doing so because he's under emotional stress, and because he's worried about losing. So take it as a compliment, and turn it into your own edge.
It's when an opponent yells directly at you that he goes completely over the line. It's one thing to raise your fist and yell "Yes!" after winning a point. It's another to raise your fist directly at your opponent while looking him in the eye. There's no real rule against this, but it's poor sportsmanship, and some umpires and referees will warn an opponent against this. The worst case I ever saw of this was when a top 13-year-old was up 2-0 in games against a top U.S. player in a best of five. It was looking like a huge upset. In the third game the top player (who was over six feet tall and towered over his opponent, who was small for his age) won the first point, walked over to the 13-year-old's side of the table, put his fist right in the kid's face, and yelled "Yaaaaaah!" I was coaching the 13-year-old, and called for the referee immediately. It turned out the referee had seen this, but rather than default the opponent as I requested, he only warned him. The kid I was coaching was badly shaken and could barely continue. He played on half-heartedly, and lost badly three straight games.
How can one deal with such a situation? It's very difficult unless you already were playing with a clear mind. Then you can look at it analytically, realize how scared the opponent is of losing, and turn his outburst into your advantage. Confidence usually beats nervous displays of arrogance.
Here are three simple points to remember when dealing with cheaters and poor sportsmanship.
Last of all, there are those who read the above, and are copiously taking mental notes on how to use cheating and poor sportsmanship to gain an edge. I have one word for you: DON'T!