Yep, I've got exactly the same problem.
At the moment, I've got a dummy spam user, and I forward postmaster to that through Pseudonyms. I then setup a rule in my mail client to delete anything in the spam account automagically.
I'm looking for a better way myself.
I know that I'm supposed to monitor the postmaster account, and keeping it is a 'good thing' (tm) but I'd like to reject all mail to it.
I'd imagine that if you go under the hood to the config files you can probably disable it, but I haven't had time to investigate.
Don.