server ip blocked because of php bulletins

[Ignacio García]

Hi there...

I'm having a problem with one of our servers. We have been blocked by 
CBL because one of our customers have been sending many emails recently 
from his php-based bulletin system. This system does not send lots of 
emails (it's programmed to send 1 email each 10 seconds), but they have 
more than 3000 email accounts in their database, and maybe half of them 
are wrong, have typos, etc (unfortunately, the customer does not delete 
their wrong entries). Also, some unexprienced recipients of this 
bulletin may have also reported these emails as spam instead of 
UNSUBSCRIBING (I've seen this many times) from the bulletin. Anyway, my 
question is:

What can we do to prevent this customer (and others in the future) to 
abuse our system this way? I was thinking of using postfix address 
verification, however there also also problems with this approach.

Thanks

Ignacio