Try this. Credit to Kim Komando web site (I think).
Problem
You run a spell check, you're in a hurry and you accidentally mark a
misspelled word as being correct. Now when you type the word, MS
Office thinks it's correct and no longer flags it as a misspelling.
Solution
I deliberately added a misspooled word (that one, in fact) to the
dictionary, then went sneaking around behind Office's back.
In C:\Documents and Settings\MyUserName\Application
Data\Microsoft\Proof I found a file called CUSTOM.DIC
This is a plain ASCII text file (ie, editable in Notepad, don't mess
with it in Word or other word processors). I opened it and there at
the top of the list was our new friend "Misspooled".
I deleted the whole line containing the errant word, saved the file,
restarted Office and once again Office regarded Misspooled as
misspelled