In a previous post, I explained how to setup a catchall with Exim. This to create multiple addresses all redirecting to the same destination:

catchall_pub:
  driver = redirect
  domains = ads.mydomain.com
  data = user@mydomain.com

Then, I wrote that on destination address we could setup a Sieve or Exim filter with a .forward file.

If you want to do that directly in Exim, to have all config in one place, or simply if your destination mail box doesn't supports filters...

Just add a local_parts option:

catchall_pub:
  driver = redirect
  domains = ads.mydomain.com
  local_parts = !/etc/exim4/ads.mydomain.com.blacklist
  data = user@mydomain.com

In /etc/exim4/ads.mydomain.com.blacklist we put all rejected addresses, one per line:

foo1
bar2

This way foo1@ads.mydomain.com and bar2@ads.mydomain.com will be rejected.

Exim4 Doc: