My suggestion: create a firstname.lastname@example.org account, and suspend it. Later, add as many aliases as needed for that account.
Worked like a charm for me, with the following return message:
Delivery to the following recipient
Technical details of permanent
failure: Google tried to deliver your
message, but it was rejected by the
recipient domain. We recommend
contacting the other email provider
for further information about the
cause of this error. The error that
the other server returned was: 550 550
5.2.1 The email account that you tried to reach is disabled.
o68si156962yhm.44 (state 14).
Only problem with that is o68si156962yhm.44 if googled leads you right to this page....