In an AIM Buddy list.
Go to iChat Menu > Preferences > Accounts and then the specific account > Security tab
In here select Block specific People and add the Screen name of the person you want to block.
If Jabber (Googletalk, Facebook or other) and if they are already in your Buddy List then Right click them to Remove them or use the Buddies Menu and De-Authorise them.
Basically AIM allows anyone to "call" your Screen name and only the Block or the restricted Allow options are the way to control who can contact you.
With Jabber they can Invite you to be a Buddy but if they are not Authorised (and then in the list) they cannot send you messages.
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
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