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.
Once a recipient name is removed, it won't appear again in the Auto-Complete List unless you deliberately select it from your Contacts list or type the address in manually. These actions will add the address to the Auto-Complete List again. If you want to permanently remove a recipient from your Contacts list, see delete a contact.