Prevent local delivery for unix accounts

[Msd]

Hello,

Is it possible to prevent local delivery for unix accounts below 1000 
(system accounts)?

I have read http://www.postfix.org/LOCAL_RECIPIENT_README.html and 
http://www.postfix.org/postconf.5.html#local_recipient_maps without success.

My problem is that spammers trie to deliver mails to system accounts 
like "www-data" with a usurped sender envelope and postfix bounces to 
the usurped address : the mailbox directory cannot be created.

Regards,


Msd