Aganju: I don't totally agree when you say that we can catch a multi nick with the comparison of the IP address, because it can happen that several members of a same family play with the same PC and don't cheat.
Example : one of the member of the family makes his moves and when he has finished, the other one logs on the site and plays, and each one has his own nick. It happens with teenagers who don't have enough money to get their own PC.