http://dryfish.livejournal.com/8148.html
The method at the above link works and is easy if you're command line savvy. A few apps may complain repeatedly, such as Aperture, but if you disable their individual spell-checking preference (usually under a "Spelling' sub-menu item) they will quiet down.
Here's part of the thread:
"Disabling spell checking in MacOS X Tiger
sudo mv /System/Library/Services/AppleSpell.service \
/System/Library/Services/AppleSpell.service.disabled
For some reason nobody in the whole internet wants to disable the spell checker. The bloody red dashed underline really annoys me and it's always on the things I say which aren't supposed to be real words and I know that they're not so what's the point of telling me they're not real words!?
Anyway, I did the mv and it's stopped so I could potentially be happy."