It's in your preferences.
In Entourage X it isn't in the preferences. Getting rid of them is a bit
tedious, so if they aren't in the way I'd leave them.
Entourage (X, anyway) keeps a database of the last 200 (I think is the
number) addresses you've sent to or received from - unless the message was
categorized as spam or the address is in your address book. Getting rid of
old ones, then, involves adding them to your address book (exactly as they
appear in the autocomplete dropdown) and then deleting the address book
entry.
What I've done is start a message and choose several of the addresses from
the autocomplete dropdown. Then I'd highlight each message and choose 'add
to address book'. Then I'd delete the message without sending it and chase
the entries down in the address book and delete them.
Tedious, as I said, but it worked.