It is less likely that a Jabber Buddy will stay in a list when removed.
AIM have a separate group that is held on their Servers of people that have contacted but ARE NOT in the Buddy list. It is called Recent Buddies and in later version of iChat and Messages you can decide not to show it but you cannot delete people from this group - and therefore the displayed list if it is ON.
However if you have more than one computer that is logged in to the Jabber server the second computer will keep re-adding (or stop the deletion) of the Buddy.
This can also happen when you are using a GoogleTalk ID and are logged in with a Web Browser to the Google Mail page or iGoogle where the Chat option is displayed.
Certain Social sites can have "Buddies" that are news and notification robots that can be added by those site having "permission" to access your Buddy list (normally with Google or Facebook accounts)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously