It's a public server, so a Custom Belt is equally entitled to join a beginner server
as a White Belt is. Of course, it can be rather annoying to consistently getting
torn to pieces by someone who is way better than oneself, so i can understand
why you are complaining.
If you ask people nicely, they might leave, but if they don't want to leave, then
there isn't a whole lot to do about it, if they aren't abusive or anything.
Remember, they're gamers as well, they also want to have fun. It's not easy to
have fun if Classic is the only mode one is good at, and if people ask one to leave.
Overall, people are more likely to comply if you're friendly. You can be pretty damned
sure that they won't move an inch if you go "WTF UR GOOD GTFO!"