I agree with *BaBi*GuRl*.
The only thing you can do is keep your head held high, and say, "It's only a rumor. Why would you believe such a stupid thing?" If it doesn't bother you, everybody will eventually figure it nots true. If you scream out, "OMG YOU GUYS ARE SO MEAN! THATS NOT TRUE!!!! STOP IT STOP IT STOP IT!!! ITS SOOOOO FAKE!", well, everybody will think it's true and your simply denying it.
As stated above, you cannot force anybody to stop. If things get too far out of hand, though, tell a teacher. So what if you get called a "snitch" or a "tattletale"? It'll be better then having to put up with a nasty rumor that has gotten to the point of bullying.