"Stop being friends with so-and-so"
It's good to know your kids' friends, but to judge their friends and decide who's good enough or not, will hurt your kid. When you tell them to stop being friends with someone, they will put up a fight, and feel as though it is a personal attack and possibly rebel. If you strongly believe that a certain friend is a bad influence, you should talk to your kid about it and try to make them understand.