Learn to spell?
I tried to rep you but it won't let me so virtual REP to you tech. Funny.
Kadio, from all I can find the only way to turn it off is as you said, per application unless someone else can find something I missed. Here is a quote from the Help system.
To turn on automatic spelling and optional grammar check:
■ Open the application that you want to use and choose Edit > Spelling and Grammar > “Check Spelling as You Type.”
■ To also check grammar with spelling, choose Edit > Spelling and Grammar > Check Grammar With Spelling.
If you don’t see this item in the Edit menu, the application doesn’t provide the Mac OS X spell-checking feature. The application may have its own spell checker. Look in the application’s preferences or menus for a spell checker.