Sorry for the long delay, I've had a lot of distractions these last couple of weeks but I've been revisiting this today. It seems that the default_forward file behaviour is programmed in the file /etc/exim4/symbiosis.d/20-routers/26-vhost-default-forward - the rules in that file tell exim4 how to read those files if they exist and what to do with the e-mail that comes through but the language it uses to describe that is a little hard to understand.
I think that you could extend the conditionals in that file so that if the recipient address matches a regular expression then the default_forward file is treated as if it doesn't exist. You could also slot a rule in somewhere else like in 21-vhost-no-local-mail or create a brand new file to filter out those e-mails before they reach the 26-vhost-default-forward part of the configuration. However regarding all of those suggestions I'm not sure how since exim4's configuration seems to be its own language.
The dumb alternative you could do would be to symlink the inboxes you don't want to receive e-mail for to a common garbage location, then run a cron job every hour or so to run something like
find /var/mail/junkinbox -type f -delete