This is just a total guess, but having done some Googling, I think I now know what you want to do. If a user clicks a button by accident, you want them to be able to 'unclick' it, yes? Is there a (right click) 'Properties' for this button, and if so, what is the value to make it 'selected'? If that is '1', could you not make the value '0' to 'unselect' it?
As I said, this is a total stab in the dark, but seems a reasonably logical suggestion.
If we can't come up with a solution for you, you could have a look at
http://www.utteraccess.com/ if you can manage to get the search facility to come up with anything remotely relevent.