You could always check out
Karma.
It may not be the prettiest add-on, but it has a lot of info and does it automatically. It integrates with your tool-tips and chat to show the karma ranking of others as you choose it. I believe it can do anything from a simple notation to a warning (I've never actually seen a warning since I tend to avoid those I give a bad rating and they are quite the minority for me...I'm generally easy-going) to completely ignore, although I've never personally used it to ignore anyone.
Ignore is usually saved for gold beggars, spammers and repeat offenders. Otherwise, I like to keep track of my "
enemies" and what they are up to. hehe
Edit: There is an add-on called
Blacklist but it hasn't been updated for some time.
It may still work with a simple TOC update. Curse.com has a newer
listing for Blacklist from Aug. of 2009.