There is a way to remove selected items but it is tedious. First, you
must know at least the first part of an address you want to remove
(normally you probably will make a note when it pops up as you are
addressing a message).
1. Start a new mail message.
2. Begin to type the address you want to delete; tab so it fills in the
To field.
3. Control-click on the blue ball next to the undesired address; select
"Add to Address Book" from the popup menu.
4. When the new contact window appears, click the Trash icon in its
toolbar to delete it.
5. Click the trash icon to delete the new message you used to get the address.
Or, if you jot down a number of these mail addresses to delete, you can
simply manually add them as contacts, and then delete them. The idea is
that deleting a contact will remove it from the "most recently used"
list, but in the case of these spurious addresses, you have to create
the contact first in order to delete it!