Ok, code the game, so if someone is cheating their key won't work for a period of time. Say a day to start, then a week. If they do it a third time, they loss there online gaming for a month. or loss it altogether.
Thats a good question. Maybe there would be a tampering program that would run with the game? Or, if you did change something (such as a mod) you would have the company filter it. Make suree it wasn't a cheat.
It's a tough call to figure who is cheating (unless it's a god cheat obviously).. I've been accused of cheating several times and I've NEVER cheated. Sometimes I'm having a good game or the other person simply misses me and they get all pissed off, accuse me of cheating and I get banned...
Maybe this is why: http://www.apa.org/journals/psp/psp7761121.html