You can't delete words from the main dictionary, but you can
create a custom "exclude" dictionary containing correctly spelled
words that you want Word/Office apps to flag as being misspelled.
See
http://office.microsoft.com/en-us/assistance/HP051892021033.aspx .
If you added a word to your custom dictionary and now want to
delete that word, then click on Tools | Options | Spelling &
Grammar tab | Custom Dictionaries. Select the dictionary you
want to edit. Make sure you do not clear its check box. Click
Modify. To delete a word, select it in the Dictionary box, and
then click Delete.