Actually, you didn't add it to FrontPage's dictionary, per se. Instead, you
added it to a custom/personal spelling "dictionary." that's common to all
Office Applications. It's plain text file which you can edit and remove
undesired entries.
The easiest way to do this is from Word (since FP doesn't have a tab - that
I could find - for this). Open Word and choose Tools, Options, Spelling and
Grammar. Click the Custom Dictionaries button. Click Modify. Find the
word, highlight it and choose Delete.