When you "add" words to the Word dictionary they are stored in a separate file (custom.dic). You should be able to edit these entries or even delete them ... see
HERE.
I personally have never tried to tell Word to use different dictionaries in different sections but I do know Word 2010 has the ability to proof different languages in different sections ... see Step 3
HERE. If the old version cannot do that (I'm not sure myself, never had the need) then it may be worth it to you to upgrade.
As for telling it to not use the custom dictionary in one section but allow it in another, I was not able to find anything on doing that but perhaps others may be able to help.