I think it really depends on the rumor, but I have found that if it isn't true and you have told anyone who asked it isn't true, then you have done everything you can. If they are still being horrible, forget them. They are losers anyway. You deserve better, and you are better off ignoring them. If it turns ugly tell a teacher or counselor. They will know the next appropriate steps. I hope this helps.
the only way to stop a rumor thats ruined you its to get the word out that its not true and get your loyal friends to spread the word and turn the situation to your advantage but the most important thing to do is to prove to yourself that you cant fall to some stupid rumor that some idiot decided would be funny to spread. take initiative and make the rumor seem like a joke and that whoever believes it is pathetic and has no integrity.
I had a rumor goim around that i had herpies and i'm only 13. Everybody came up to me at the fair and prettt much said im a whore an i had herpies. Even people i didnt know. After awhile i got sick of it and told people that i didnt care. It didnt bother me what they thought because i knew i didnt have herpies eventually the rumor went away.
If you can, find out who started it and confront them with a school counselor present. If that does not work, you can just wait it out. If they do not get a reaction out of you, it will usually blow over after a bit. You can also do the opposite to what the rumour says, if they say you are gay, start dating a guy to prove them wrong. Another way to deal with a rumour, is to laugh at it to show how silly it is. Ultimately the person spreading the rumour will stop, if they cannot get a reaction out of you. Good luck! : D