Plug-IN

How do I get rid of a rogue PLUG-IN? I am trying to get rid of a Twitter Plug IN. It is not showing as ACTIVATED on the list of Plug-Ins. It is not showing at all in fact, but it was there at one time. When I click the Twitter button it goes to this person’s twitter that is not mine. It shows my toolbar and information at the top of the Twitter, but their Twitter page. HELP!