alex_w which would suck if the person you were ignoring only had one contribution (the OP) and the rest of the discussion was from non-ignored members.
So far, I've not needed to ignore a person. But what would probably be useful is if each thread had an ignore option, which would prevent a member being shown any new posts in the thread.
Here, I'm thinking of incessant weekly new threads on subjects like rollover. The replies to these cannot possibly be saying anything important that hasn't been said before. So when yet another rollover thread appears, we can just mark the thread as ignored. Then that thread would be omitted from the relevant indexes and possibly from notifications too.