Personally, I would probably only use an "ignore" / "block user" button if it could be applied per thread... unless I noted a specific individual doing nothing else, ever, but trolling or evangelising obsessively and aggresively on one topic, I suppose...

...but, the technical implementation of per thread blocking would likely be much trickier, I would guess.

Even if I wouldn't use it, I guess a more general "ignore user" button would be welcomed by a lot of folk though...