pedestrian: The multi-nick (or 'cheater') topic has been discussed a lot, also on other game sites.
On the server end, there are some pretty efficient ways to catch them:
1. IP address comparison.
2. Whenever a user logs on, store the IDs that logged off in the previous three minutes. After ten times, if the same ID is in the list too often, it's with high probability a multi nick.
3. Compare the lists from 1. and 2. with who plays whom. Same user IDs come up, there you go.

You can imagine that with some fiddling and tuning, this would be rather efficient. Unfortunately, you need to have server access, as the IP address for example is otherwise private data.

The next question would be - what do you do with multi-nicks? Delete them and lose all running games? Remove any positive rating efffects from playing between them?