What to say to someone who is saying rude things about my family?

[alicia]

There is a girl , and she is always bragging about herself. She lives the next street over from me, however I haven't seen her around much. She hangs up on the phone when she hears my friend Anne... no idea why. Anyways, she is always being rude to my cousin : calling her stupid, poor etc. in front of other people. I really want to tell her off, but I need to know what to say. If I say anything bad (like a bad word) she will tell her mom, and she will tell all of the other neighbourhood moms who will be mad at my mom and I don't want that. How can I say (nicely) to buzz the f*** off!? But still make my point??
Don't think it should be in friends. She isn't my friend, and won't be ever again. I always thought she was rude, and I just want her to stop being rude to my family, I don't want to speak to her again.