Should have looked a bit better before posting, and appear to be unable to edit/delete my previous post.

The solution is quite easy: add

local_parts = !/etc/exim4/domain.com.blacklist

to the custom router, then create the file /exc/exim4/domain.com.blacklist and add the to blacklist addresses there, each one on a new line. e.g.

# cat /etc/exim4/domain.com.blacklist

noreply

spam

bar

Ajenti's built in text editor with a bookmark to the file makes adding blacklists items convenient.

Thanks, works great, but is it possible to enter exceptions to the catch-all? Say I want to block anything coming to noreply@domain.com