%ProgramFiles%\Messenger Plus! Live\MsgPlusLive.dll I think is the main DLL that hooks onto messenger. I'm not too sure but since whatever file it is is a DLL and therefore isn't shown in the process list, GDI++'s ignore list might be useless in this situation as GDI++ likely checks for the filename in the process list. The best you can do if this is the case is set it to ignore the whole of Messenger.
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