It can't be prevented. Whether it's two accounts, a couple squaddies working together, or players destroying unprotected stuff, there are going to be players padding their scores. I don't know if there's a game on the internet that doesn't have the same problem. The only logical result is for players to take all scores and ranks with a grain of salt. Use relative scores to judge personal performance or comparison between trustworthy players. In the end, if you come out on top in a battle, even if you didn't kill your opponent, you know you're the victor. Scores are secondary.
Imagine a player with two accounts spending the entire afternoon killing his other icon over and over and over again. Who's the loser? Laugh and move on.
