If you are using AllMail, you may find that certain addresses get a disproportionate amount of spam. If you find that one of your addresses gets a huge amount of spam, you can exclude that address from AllMail. (Messages to *all* your addresses, AllMail or not, are filtered using the same spam settings. I personally will remove an address that seems to have become popular for spammers simply to reduce the number of spam messages I need to check.)
Just go to Manage Your Domains, and select the domain of the address in question. At the top of the page, click "Ban Address", and type in the address. Once you add the exclusion, Pobox will immediately begin rejecting mail for that address. Users who send mail to that address will receive a bounce message, with the error "User Unknown".