Never. Not even tempted to. I think the subjects are what get ignored by me. I recognized a name on an innocuous thread who I really can't stand. Twice I caught myself responding to some movies this person was mentioning, and I stopped myself. I wasn't going to get irate, I just don't want to interact with this person after all the garbage I had seen.
I think that's a good attitude. I haven't hit ignore on anyone but there are times that if I find myself typing out a response to something that irritates me a bit I just ask, "is this worth it?" Usually it's not and I hit refresh and move on. Getting better and not even getting to the typing part. :)