In most of the Cocoa-based applications, you just right-click in the text-box and select Spelling -> Check Spelling as you type (and make sure that option is un-checked).
I don't see that option in Camino however, so presumably they rolled their own text box control. I also see no option for turning off spell-check. Frankly, I think having spell-check built in to nearly every OSX application is one of the (immeasurable) things that sets the Mac way ahead of Windows, so I can't imagine turning it off either (unless I was using a web-interface for building code, in which case I could see it being quite irritating).