[Exim] "User unknown" for spamprotection

Hi,

I use sender_host_reject_recipients to reject some spamsites, and now
I am looking for something on userlevel.

Is there some way to generate a "user unknown" kind of error with
.forward filters?

Maybe the "mail" command can be used, but is there some easier way?

I hope this is will cause the sending site to stop sending (like some
announce list from some big companies which are too dumb to properly
unsubscribe someone).

Ciao,
    Martin