Agree with Nojo and Math here.
Perhaps it is because I only play in rated or forum organized games, but it seems to me that the extent of multiboxing in games is greatly exaggerated. If you wish to avoid it, buy L2! It also has the wonderful benefit of supporting this wonderful game!
Multi-game alliances are a completely different problem, in my opinion. They are sometimes irritating, sure, but they are still beatable. However, as I said in a different thread somewhere, in the game I played with Lord Akkrand against a pre-set 3 man team, that "team" did quite poorly and I won the game.
Diplomacy is a powerful tool.
That said, I agree - if you see a few players always in the same games...bring a few of your friends in there, too. Make initial peace deals with them, then stab them in the back immediately.
Subterfuge is also a powerful tool.

"No man chooses evil because it is evil; he only mistakes it for happiness, the good he seeks."
-- Mary Wollstonecraft