I figured I'd make a note about spell-check and move this thread to the forums feedback board.
Anyhow, here's the deal with spell-check. Currently, if you use Firefox, it will continue to check the same word over and over again. However, Firefox has a built in spell-check. It underlines a word with red dots if it's spelled incorrectly. If I don't know the true spelling of the word, I always just check dictionary.com for the correct spelling.
If you are using IE, spell-check should work correctly, but it will check *every* word within your text box. If you quote a post, it will check the words in there because it's technically part of your post. I usually just ignore the checks within a quoted post and start paying attention when it's gotten to the body of mine. The only way around this at the time is to write out your response, spell-check it, then hit the "quote post" button after your spelling has been checked. It's a little extra effort, but that way you don't have to sit through the spell-check for the post you're quoting.