What about adding this to main.cf?:
inet_interfaces = loopback-only
I believe that that would work as well. Basically, the idea is to cut
off outside access to postfix so that new messages don't get in the
queue. Simultaneously, postfix is attempting to empty the queue as
normal. Eventually, anything in the queue will either be delivered or
will time out and generate a bounce to the sender (after
maximal_queue_lifetime). Any bounces will eventually timeout themselves
(after bounce_queue_lifetime). This may take a few days with the default
settings.
