Your 10.6 Spaces key gets redirected to Mission Control automatically, but there is no option in System Prefs to unbind that key or cleanly reassign it.
To do that, you'll need to edit your symbolichotkeys plist:
defaults write com.apple.symbolichotkeys AppleSymbolicHotKeys -dict-add 75 "{enabled = 0; value = { parameters = (65535, 96, 0); type = 'standard'; }; }"
defaults write com.apple.symbolichotkeys AppleSymbolicHotKeys -dict-add 76 "{enabled = 0; value = { parameters = (65535, 96, 131072); type = 'standard'; }; }"
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110721132409362