Like you said, people like to see the reaction of whoever they are bullying, so try not to show them that the situation is upsetting you. If you keep a calm mind around them, it should take away what they are craving from you (a reaction.) Of course, this will take some time for them to get the message, so in the meantime, you could tell whoever it may concern that the rumours are false. Tell parents, teachers etc. about the problem and just remember that a rumour, although often highly distressing, will usually go away on it's own, in time, and be forgotton about when the next bit of crappy gossip pops up.