You can ban that person from the game.
You can modify the game to make that method of cheating harder or, possibly, impossible, but still leave the desire to do so.
You can modify the game to make that method of cheating undesirable or useless.
The first is by far the easiest, and also the most ineffectual. The third is hardest, but is very, very effective.
Topcoder seems to be doing a lot of the first, very little of the second, and none of the third. Most MMORPGs are doing the first and the second (although, to be fair with them, the third would be much much harder just due to the nature of the game.) Most board games have the third nailed down pretty well, simply because nobody wants to cheat because they'd get booted from their circle of friends.
Things to think about.