Christa Connell wrote an informative article, "When you don't like your child's friends". It's a good place to start if you need help dealing with the issue. Psychotherapist Andriana Mantas reminds parents not to criticize the friend, but the behavior. Be sure to ask your child what they think of the behavior, you might be surprised to hear your child has already identified the problem. Although you can steer your younger children away from a bad influence, Mantas argues that you shouldn't cut your older child off from a friend you don't look. You might want to more subtly limit the time opportunities your child has to spend with the friend in question.
Finally, keep the line of communication open with your children. Mantas suggests, while guiding your children, also listen to their opinions and give them the freedom to make their own decision about their friends.