Aside from some weird trolling-type messages way back, there’s only been one PM that was really hostile, and I didn’t answer it. I’d have called upon the mods if there’d been a persistent problem.
As for not having a blocking option, I believe the answer to that is the same as the answer to a lot of other things, namely, that development on the site ended when Ben and Andrew moved on. A block feature (for posts rather than PMs) was at the top of my wish list 5 or 6 years ago, but I guess it wasn’t a priority. By now we just have to be satisfied with what we’ve got, which I still think is balanced in our favor. The fluthergods did a great job in the first place, and the mods stll run a pretty clean shop.