I don't like killfiles and /ignore mainly due to the fact that it's very confusing to see only one side of a conversation afterwards. I'm using some kind of "greylisting". For email, I have a procmail recipe that looks like:

# killfile
* !^Status:
* ^From: .*<(noob@bla.com|luser@foo.org|troll@evil.net)>
| formail -i"Status: RO"

This will mark the messages as read but leaves all threads intact. (I'm using mbox folders.) That way, I'm not bothered to read the messages unless I really want to.

In irssi, I have a /hide macro:

hide    color set $0 15 ; color save

Together with a patched version of nickcolor.pl, this will color the nick in light grey. Since that is hardly readable on white background, I can only see which troll is talking there if I have a close look.