Sounds like disconnect supervision isn't working. Does your telco provide DS, and are the trunks configured for it?
There was another thread in the Norstar forum that suggested having your AA default to an info mailbox instead, that just says "thank for calling" and hangs up, if you can't get DS working. That's an option, but then anyone calling from a rotary phone (all 2 of 'em left in the world) would never get through.
- Adam