[...] without the complication of a full client-side kill/score filing system.
We're not talking about client-side. Rather, another database table (or two or whatever) on the IWT side, User Preferences editability for it, and of course, the functionality to remove/restrict those posts by twits from your visibility.

As far as your comment about a Purgatory/rating system, I would view that as *more* work than a system-side twit filter/killfile system.