You do your best to raise your child with your values, surrounded by positive influences and protect him from harm. No matter how successful you are in this quest, however, your child is bound to encounter a classmate or social friend whose behavior you don't like, or whose family doesn't share your rules. Handling this situation correctly is crucial to preventing the unacceptable friend from wielding too much influence on your child. Stay calm, talk to your child in a way he'll understand and don't overreact, but demonstrate to your child that, as the parent, your most important job is to keep him out of harm's way.
Talk to your child about his choice of friends. Ask him why he wants to spend time with the kid you consider a bad influence -- what is it about the child that makes him so appealing? Refrain from labeling the new friend as "bad." Focus instead on the behaviors you don't like, and explain why, in terms appropriate to the child's age. Tie your comments to well-established rules or accepted behaviors in your family. "Bobby says lots of bad words and mean things to other kids; that's not okay in our family" is appropriate for a young child, for example.
Limit the time your young children spend with the questionable friend. You still have considerable control over their social interactions and activities, so it's much easier to do this when your kids are young than it is for preteens and beyond. Don't let your youngster go to the other friend's house. Instead, restrict their time together to short, supervised play dates at your house, so you can monitor their interactions and redirect their behavior if necessary. If the new friendship is causing problems at school, enlist your child's teacher in subtly separating the children when possible.
Take a more measured approach when dealing with tweens and teens. At this age, telling your child you don't like one of his friends might just enhance his appeal in your child's eyes. And outright forbidding a teenager to spend time with an unacceptable friend all but guarantees he'll try harder to do so. Encourage your teen to spend time with more acceptable friends, or at least limit his one-on-one time with the questionable influence.
Tell your older child why you're concerned about his new friend, without sounding as if you're questioning his overall judgment. Reassure him of your trust, but make sure he understands that you don't want him caught up in harmful situations or hanging with people engaged in unacceptable behavior. If he responds with the standard "Mom, he's not that bad, you don't even know him," call his bluff. Invite the friend over for dinner with the family and spend a little time trying to get to know him and making him feel welcome.