On the sidebar, go to Mailbox, then Buddy List at the top. There's a little x on the far right of every name on the list. Click the x to take the person off your buddy list. :)
I did just that and the name isn't there anymore, but on the little button on the top ( you know the one that says "1 friends online") still shows that user as being online.
EDIT: never mind, it must have taken a couple of minuets to kick in. All fixed. thank you.