I wouldn't do anything. They are kids, and just because she is an ICP fan, Juggalo or otherwise, doesn't mean that she is going to turn your son into a drug abusing alcoholic.
Challenge her and her Juggallo friends to combat in the roof of the Whitehouse, cleaving them in half with your katana before escaping on a flying Skateboard.
When he brings her home for you to meet her, greet her wearing full Juggalo makeup. Pretend to be a huge fan, and speak to her and your son in such a manner. Be as lame as ******* possible. At the very least, your son will be embarrassed enough not to bring her crusty ass around the house anymore.