Re: What to do ?
I am going to throw my own thoughts into this arena and say if your not comfortable training with person B then do not. If your Sensei knows the circumstances around what happened and why you do not wish to train with him then he should respect that. I would just have a quick chat with him, after all if he was banned before then I assume he does know the circumstances around it.
I would like to think if this did come up then a solution could be reached. If you really do not want to train with person B then don't.