Blacklister (Beta) now available at http://tinyurl.com/nhzeftx
It's still fairly basic, but we like the idea "release early, release often", so that we can gather valuable feeback from users. :)
Since comments seem to be filtered by some sort of censoring/antispam, let's try again: I thought I'm working on such plugin. I'm not sure how complex it will be to implement it, but any feedback and suggestion will be welcome.