After setting the keybindings back to disable did you restart gnome?
It seems keyboard shortcuts get placed in at least two different locations. Luckily, they are under the same parent directory, though: .gconf
Open a terminal and change to:
You should see a file in there: %gconf.xml
Either delete this file (you loose all your custom keybindings set in that file) or edit the file and delete the settings you don't want. Then restart gnome (log out).
I have also seen keybindings ending up under:
Again, either edit or delete the %gconf.xml file in there.
Keybindings may get set in other subdirectories, I did not test all the settings. I would suspect, though, that for Gnome, they should all be under the .gconf directory. Focusing on those two locations above should get you back in business.
Worse case scenario, move .gconf to .gconf.old and restart gnome. This will wipe out all your settings and you go back to defaults. From terminal:
mv .gconf .gconf.old
If something goes screwey, you can always restore your .gconf by moving it back.