[Tess*]

Tell them it's not funny. Tell them it bothers you.
If they're true friends, they'll stop.

I guess if they don't stop, dont' react when they make a joke. I know that can be really hard. But ya, don't say anything, don't look at them, don't even change facial expression. That's pretty much what they want, some kind of reaction from you to keep them laughing.