[exim] limiting messages send by certain users?

Hello,

we are hosting websites of several thousand customers and a few of them
are obviously using insecure formmail scripts. While it's impossible to
get rid of all formmail-scripts, I'm thinking about a solution within
the mta (exim) to limit the messages that may be send within a certain
period. If someone sends more than 1000 messages a day it ios very
likely that someone is using a formmail (or similar) to spam aol and
hotmail-accounts. Do you know of any example or howto to achieve this
with exim? The question is also: What should happen with mails above the
limit?

Regards
Marten