Since I experienced massive misunderstandings by quoting single post URLs that get substituted by the thread name I´ll start a new thread with this topic:
Unvote doesn't decrease vote count
Pligg logs the votes and their value of every user therefore it should be possible to offer an unvote link at user.php?login=USERNAME&view=voted.
If it´s easier: An admin link would be enough for me.
Edit: Since it uses the same table it could be also modified to an unvote at the comments domain.