I started out with nickcolor.pl, flirted with a patched version of nickcolor.pl (see: Killfiling on Myon’s Blog), and ended up with a rewrite in the form of Greynick. It’s a script for Irssi that implements something like ‘/ignore’ (but nowhere near as flexible), but colours the text differently instead of not displaying it. In my case (and currently hardcoded, sorry), it’s dark grey (bold black), on black, giving it less impact than normal text.