Removing an icon from Mac OS X menubar when Cmd+Drag won't work
If it is opened at login, check login items in the Users &amp; Groups preference pane, {~,}/Library/Launch{Daemons,Agents}/, or /var/db/launchd.db/com.apple.launchd.peruser.501/overrides.plist.

You could also try opening Activity Monitor and quitting all processes that seem suspicious.