Again I can't be specific because you haven't said which version of Windows you have, but the Spelling tab of Options is where you can control this. Also be aware that you can press the F1 key for online help.
Every time I try to send an e-mail that has my last name or some abbreviation WLM doesn't like, it will give me a correction selection box. I have to click ignore, then the box that comes up asking if I want to send it without the spell check.
I'd like to stop that--I don't really care if I make an occasional spelling error, but don't like to spend the few seconds every time I send an e-mail.