Other sites that have ignore functionality:
Fatwallet
Slickdeals
(I'm sure others can add more)
Ignore Thread/Forum add-on (unofficial) for Vb:
http://www.vbulletin.org/forum/showt...d+forum+ignore
(But it's apparently EXTREMELY limited, in that it only blocks them from the "New Posts" view)
Threads still show even if you've ignored the user: Known bug in Vb.
http://www.vbulletin.org/forum/showt...=ignore+thread