What can you do to make your chile understand that some people is not for them

If they won't listen... let them fall, then pick up their pieces if you wish. It's the only way they'll learn...

U cant. U let them learn by there own mistakes and just be there for them to help pick up the pieces xx

Unfortunately, the previous answers are all correct. All you can do is give them the best talk that you can, and then go on blind faith alone that your kid will make the right decisions. Even if it doesn't seem to sink in, I remember when I was at my worst. It was my mother's voice that always kept me leaning toward the way I should go. It eventually sinks in.

What is the reason that you don't want your kid to hang out with these certain people?