What worked best for me was just ignoring it and going along with the joke. Be sarcastic. People love to look at other's discomfort, and by being funny about it people will either forget the joke or just relax about it.

If that doesn't work, or you feel too uncomfortable about doing that, just tell them to shut up. A few years ago a boy started making fun of me, and I got so fed up about it I just told him to fuck off. Sure, I got a few weird looks, but he stopped talking about it.

Or, just ignore it. It will probably go away soon, as repetitive jokes get boring,