Q: How to remove Spaces mouse button assignment?

I accidently set the "Activate Spaces" button to mouse button ~200.  And now whenever I click with my magic mouse, anywhere, it activates Spaces.  I can't figure out how to tab through the menus to try and change this or reset it to nothing?

I can't even figure out which OS version I have, but I'm pretty sure its 10.5 something.