As Janet said above, this is a speech to be given from your Sensei. If she/he wants you to give it, make sure you both are clear on the "...or else" part.
I would say...
"It's nothing personal (ok, maybe it is, but lie) , but I need to talk to you about your training here. If you want to endanger yourself, fine. Go ahead.
But every time you are on the mat, understand that you are a serious risk to the person you are training with, and an equal risk to those around you. Whether you choose to not listen to what you are told/shown by Sensei (or myself or the other sempai), or are trying to and still unable is of no matter.
It (enter list of issues here) has to stop. It will stop. I cannot allow this to continue. Change or you will be banned."
Think of it as a verbal atemi
For those who do not listen to what Sensei has said, you could choose to be the enforcer; be your usual self with everyone else, but really lay it on 'em (someone say "How un-aiki"?) when you train with them. This could backfire, however, and encourage them even more.
Or simply do not train with them anymore, which is my preferred tactic.