I remember a friend of mine, with a complete set of 6 sided dice, each one with two six and no 1. He was caught using these to roll his stat.
He was caught doing many, many things like this. As we play in a club, we have a lot of DM and PC, and we played a lot of one shot. Some players were reknowed for gaining level, stat or magical item/divine gift between sessions...
The same player came one day with a character which had *gained* "total and absolute immunity to detection". He was undetectable. (In fact, he was jealous of a wizard who abused invis/non-detection some days before). So the DM said OK, and then, secretly, asked the other players to absolutly ignore him.
At the begining, the player was happy, cause he was spying everybody, without being detected. But when he tried to join a conversation, no one cared for him.
"You are undetectable" said the DM.
The player asked "what if I strike them with my sword ?".
"They will be dead, without knowing why : you are undetectable".
"But what can I do ?"
"Create a new character".
Sometimes it works to let the cheater cheat, and caught them in their own game : they quickly realize there is no fun. Another (Or the same, I'm not sure) was caught cheating with his roll : for the rest of the session, each time the player make an attack, the DM said "stop, don't roll your dice : you just score a critical hit : your ennemy is beheaded". The more frustrated was the cheater, of course. Now, this player has learned the lesson, and he doesn't cheat anymore.