How can I remone an old keyboard shortcut

OK - I was stupid. But green and stupid.

A long, long time ago - when Expose was first introduced I found it so useful that I changed the default shortcut key to just use the 'fn' key. As I am not a big user of shortcut keys this worked just fine for me.

But the problem is that it still works just fine although it is called Mission Control now. And now, of course, I want to set up some other shortcut keys but can't set or change anything with the fn key because as soon as I press it - Mission Control cuts in!

And I just can't find how I can remove this old, dead, shortcut. Even changing the shortcut to Mission Control itself does not kill it.

Can anyone advise on how and/or where I can get rid of this product of my more youthful exuberance?