The history of your responses is kept in a local cache file which you
don't get to edit. You will have to purge ALL stored credentials so you
can answer Yes when you get to the site for which you want to cache
those credentials (because now that domain is no longer in history to
see that you said No before). Yeah, sucks.
Well, there is no handy edit function that will let you select which
cached login credentials to remove regarding your history. However,
autocomplete is based on forms, not on just the login credentials; i.e.,
the cached credentials have relevance within the form in which there
were used and which got recorded by autocomplete. Go read
http://pubs.logicalexpressions.com/Pub0009/LPMArticle.asp?ID=348 on a
workaround (Google works and the above article was found using
http://www.google.com/search?q=+autocomplete++"internet+explorer"+%2Buser+names+%2Bpasswords+edit+remove).