Although it would perhaps be little used by many, there are times when it would be good to be able to blacklist a member if you're not happy to respond to them, or you don't want any notification of their involvement, or you don't want them to be able to @ ping you.
Example is the other day, a member on this thread...ERP designing help me Steps and proper flow for ERP (which he's now edited and it's been moved to a more appropriate space) had included a long list of @ pinged members, oracle employees and other community spaces, effectively requesting the attention of all those people and anyone following those spaces.... all for what? just his little question... regardless of how relevant it was to any of those people. I'm guessing he had just gone through the various lists of Top Participants and thought he'd get the attention of all those people as they are 'experts' or 'gurus'.
That's just one example, but there are others that have been seen where there has been heated exchanges between members who vow never to assist that person again. As such, it would be good if we could have something in place to effectively ignore that person, and should we come across one of their threads/posts, some indicator that they are on our 'ignore' list, so we know not to bother getting involved with them.
I know this seems like a negative aspect of 'social' interaction, but it's something that is included in a lot of other forum software, and has it's benefits.