As unpleasant as it may be, there is really no way to keep him from saying nasty things to you if you break up with him again. In fact, he's likely to do the same thing he did last time. But this time you will be prepared and you can choose to react differently.

This time, if you are truly serious about breaking up with him, don't take him back. Sticking to your guns is the best and probably the only way to communicate the message that you mean what you say: you're not interested in being with him any longer.

Good luck.