[exim] Using Exim ACL to blackhole bounce messages

Hello,

First of all, I'm sorry I want to abuse Exim and the RFC's in this way,
but I'm kinda out of other ideas.
At the moment a lot of domains which I use Exim for, receive bounce
messages to their catchall's, which are bounces from spammers.

So spammers abuse the domains of some of our clients as 'from' addresses...


My idea is to try to find a way to have Exim's own ACL detect bounce
messages and automatically blackhole them, without any further
processing. This way clients will not be able to receive bounces,
however for the time being, I find this to be a good solution to the
problem. This due to the fact the spam filter SpamAssassin has already
caused overloads on some servers.

Do you perhaps have an idea on how I could implement something like
this? - I'm a bit of an Exim ACL newbie...

Thank you.