Its simple realy:

1. You give them feedback in a respectful manner.
2. You offer advice on how they can improve.

If they improve or you think they are doing their best, I would let them stay. If they are not listening to your feedback and still underpreforming, then I kick them out. No drama, just a kick and move on. This game relies on teamwork, everyone should try and give it their best when it comes to clearing content, no matter what content always give your best.