
Gonzo: Permit multiple accounts and secret diplomacy (which is almost as good as having two accounts in one game). The game then devolves into a meta-game of secret deals, backstabbing, and suspicion. But at least it's a meta-game that everyone can partake in equally.
Totalitarian: Remove fog of war (the multi's best friend), forbid any teaming up, open or not, and add the capability for game replays. Then even a person with two accounts in one game won't be able to gain an advantage from them, since they can't have their accounts work together. But, since teaming up is necessary in some circumstances, and many see teaming up where there is none, this will lead to huge headaches for everyone.
Restrictive: Only count 1v1 (better: 1v1 triples or quads) for points. You can't abuse multiple accounts if there are only two people in the game. Of course you can still dominate the no-points games, but, whatever.
New Rules: Create a rules variant where skill matters more than numbers. The primary reason that having two accounts in the same game is abusive is that having double the troops is a huge advantage. This is primarily because the strategy of the game is extremely thin (not non-existent, thin). The real skill involves diplomacy (both in chat and by positioning yourself so that you don't look like the biggest threat), which being a multi also helps

Or you could prove me wrong by busting all my accounts.
