lol yes you definitely need the dictionary. "Costume" is what people wear
You could always just turn the spell check feature off. And, and you mentioned, you can use Auto, I don't see how that defeats the purpose, it's like a "live time" spell checker.
Why don't you just add the word you want to the BlackBerry's dictionary?
In a Message list, press Menu, select Options, then select Spell Check, and click on Custom Dictionary, press Menu and select New. Then just type what you want and save it.