As much as I used to want to have this feature here, I don’t think it’s necessary or warranted in Fluther,so I’ve changed my mind.
Like @Jeruba, I have also been a member of another Q&A site that had a “block” feature. Although some posters there annoyed me from time to time, I never availed the feature to block anyone; I learned to ignore, bypass and simply not read the things they posted, or just laugh at their antics. (Had they attempted to annoy or harass via PMs, I might have changed my mind.)
On the other hand, when I finally crossed a line with one of the thin-skinned ones there by commenting too caustically once too often, I found myself blocked by that person. It didn’t particularly bother me, but since it prevented me from responding directly to that person’s posts, I had to POP them (post-on-post, where we can “adopt the question” and post about it without responding). I found that using that feature made me even more sarcastic, which isn’t one of my better features (even though I can do it pretty well).
For all my complaining about the mods here from time to time, and there is no doubt that they occasionally play favorites, over-mod those they don’t like, and under-respond to complaints from those people as well, learning to “play nice enough to pass within the rules here” has actually made me a better writer – even if a majority in many threads still disagree with me. So I’d generally vote against a “block” feature.
The mods do tend to keep down abusive and personal attacks; I’ll definitely give them credit for that. (I don’t know about PMs, though. I don’t get much “hate mail”, as some have reported from time to time. That might be block-worthy, but again, I think the mods generally deal with that, from what I’ve heard.)