My previous full time job had this approach too. We sent out a lot of custom designed spam, and if someone unsubbed or asked to be removed via reply we blacklisted them so hard they literally no longer existed in our universe. This was an order from high up and always seemed a bit dubious to us - but when comparing approaches, I’d much, much rather have this full on blacklist approach than be forwarded to another spammer, or face double opt out procedures.