[Exim] fail command only available in system filter?

I want to use "fail" in something other than a system filter.

Here's what I want to do. There are spam sites that I can determine with
100% accuracy that some messages are definitely spam. What I want to do
is to fail these messages as if the recipient doesn't exist - in the
hopes that the bounced message will cause people to be removed from spam
lists.

I can bounce them at the system filter level - but I want information in
the envelope-to: headers that don't exists at system filtering time. So
i'm using a user filter to catch this spam - but it won't let me fail
the message.

I don't know if there is a work around but I'd like to have a command
that allows me to have the fail command in user filters.