I just copied this from an article that googled up:
"If you want to remove that incorrect entry from the recently used list in Entourage 2008, create a new message and start typing the address you want to remove. When the pop-up menu of suggested addresses appears, select the incorrect one, hold down the Control key and click on the address. Choose the Add to Address Book from the menu to create a new contact.
Now that you’ve created a contact file with the incorrect address, select it, go to the Edit menu and choose Delete Contact. Once you delete the contact and discard the blank message you created, the entry should disappear from the Recent Addresses list. (If this is an address of someone you have begun to regularly correspond with, you can also just correct the information in the new contact file so you have right address in your address book.)
If you want to wipe out all the addresses on the entire Recent Addresses list, go to the Entourage menu and choose Preferences. On the left side of the Preferences box, under the Mail & News Preferences, click on the Compose line. On the right side of the Preferences box, in the Recent Addresses area, click on the Clear List button."