You can remove one or more entries in the Recent People list. If you hover your mouse over the entry, you'll see an X to the right of the person's entry. Click that X to delete the person's name and email address from the Auto-Complete List. This doesn't remove the person from your Contacts folder or other address book.
You can also use the arrow keys to move through the list, and when you reach the person's name that you want to delete, you can press the Delete key.