L.S.
You are in for a long hard battle with this problem. I have a now 15 year old, and she has gone through 4 stages of this problem.... 1st-2nd grade, middle school, and now high school...
We can't decide who our kids play with, or who they are friends with. We can only limit their access to the bad influences, but sooner or later, they will have to learn to make these decisions for themselves. The kids I tried to stop her from befriending, she just talks to them at school, where I have no control.
It is a hard thing to handle, you know these people are no good for your child, yet until they realize it, you just have to sit and watch, and be the leaning post when the friendship fails.
Sorry to say it... there really is nothing to do except to keep re-teaching your own child what is right and wrong, and how to move away from these people when they decide they have had enough.
Good luck M.... I know exactly how you feel!