Let me first say that I feel for you! That simply sucks.

My sound engineer says that this sort of thing is pretty standard fare in many groups. It is truly like a marriage, and sometimes you love the kids (the music that you produce) *so* much that you stick it out even when the rest of it is horrific.

All I can say that may be of use is this; gently stand your ground, and lay down the boundaries (and I am not saying that I have always lived up to this. The heat of the moment has gotten to me many a time). But if you can, keep your cool and let the person know you are not going to tolerate it anymore, tell them what will happen if they cross your boundaries, and then stick to your guns. It does not have to mean the dissolution of the group. It may mean the dissolution of much of your communication with this person beyond that which is absolutely necessary.

In my mind, when things have devolved to that point, it is a business relationship only anyway. I would not allow a co-worker to abuse me so. Why should this be any different?