In the FAQ thread near the top of this forum, there is a long explanation from Lau (a former mod here) on a method to block users by downloading certain add-ons to your computer.
He posted this a fair while ago, so there may be newer/better/updated versions of the same add-ons now.
I haven't tried the method he describes. It would be interesting to hear from anyone who has successfully used such a method to block members they find continually obnoxious.