When you create the policy, rather than adding users to the policy, you select the "Add Anonymous User" button (see screen shot). To expire the document at a certain date, set the document validity period (General Settings in the policy).
If you want to do this with the currently protected document, you will need "remove" the policy and apply the new one that is configured for "Anonymous" users.
Regards
Steve
P.S. This is posted in the wrong forum (it should be part of Rights Management, not Reader Extensions)