I have felt that kind of hatred before and for quite awhile it practically consumed me. In my case, the person is long gone out of my life, but I knew then and still know, today, that he is out there creating havoc and pain for other people. Unfortunately there’s nothing I can do about it that won’t ruin me financially or make me end up in jail for murder.
So what I had to do was stop thinking about him in a manner that made me constantly wonder how I could get him or punish him (even though he deserves that). Now I try not to think of him at all or when I do, I try to come up with other images and I definitely have to come up with positive activities to occupy my mind when thoughts of him pop up.
And the other thing I have done is to try to live my life in a “Pay it Forward” kind of way. Even though I can’t change him, or stop him from behaving like he does, or ever repair the damage that he did, I can do things to make the world better for others, even if it’s just in small ways.
In your particular situation, since the “problem person” is not likely to go away any time soon, you need to come up with ways to limit your contact with him in ways that you can still function the way that you legally have to with him. That may be by only communicating by e-mail as some of the others have said, or to pick an intermediary (maybe a lawyer, a strong friend, or maybe the others have some good ideas for whom could stand in as an intermediary). Then make everything very businesslike with the person as much as you possibly can.
For you own sake, come up with every scenario you can think of and brainstorm with yourself, your friends, us, and even your lawyer, and come up with solutions and ways in which you will deal with or handle these situations when they arise. It’s so much easier to know exactly what you are going to do, if you have a plan in place (with a couple of contingency plans in place in case the first thing doesn’t work). It’s so much easier than simply having to come up with a plan on the fly. I think that’s the worst part for you, at this point, is that you keep getting blind-sided by this person’s actions. So come up with a ridiculously long list of situations that he might pull out of his hat, and then come up with lists of ways that you will deal with each of them. Write it out, don’t just think about them. Put your ideas down on paper and read them over occasionally so you’ll know what to do when he does X, Y or Z.
I also highly recommend talking to a therapist, simply for the support they can give you and for the mental tools they can give you to help you better cope. At this point, you probably can’t do it on your own. Will power alone is not enough, you need a strong advocate (the therapist) who is not invested emotionally in the situation.
In the meantime, let’s hope that he somehow becomes interested in sky-diving, or hiking alone during the winter time, or motorcycle riding or flying experimental aircraft. Nice safe activities for him : )