You may also consider idea I have put into practice with a previous (almost ten year old; now closed) community I ran–
Make the admin account a strict “root/utility-only” account and then make a Leader-level account for yourself. Now you can disable PMs on that Leader-only account. Admin account becomes the “if I need to send a PM for technical reasons, then here: not related to conversation and/or business dealings.” And those who insist on sending you these “just want your small opinion” PMs to your admin account shall go unanswered–as long as you make it clear the admin account is only as a tech-support channel; reporting issues RE: using your site or Discourse instance et al.
That sounds like they know that too and are taking advantage of you from that: they are previous clients, paid for some classes already gone and archived, or some freelance. That does not give them carte blanche to pinch free intel off you because of a past that is open and closed; paid and done.
And if they get upset, learn the lesson I learned long ago: you are not going to please everyone. After you accept that truth, you get more respect when you stand up for yourself, even though from that you are now not necessarily liked first. I–from personal experience–prefer that. Respect be business gold versus the lowly pewter: only being liked or seen as “nice”. And thus seen as possibly a pushover.
(Apologies; this second suggestion does not really attempt to “answer” your technical question. It does offer insight into a larger situation I see, that this technical question only seeks to avoid or hide from and ultimately be part of your paid membership biz model–and a weakness in it some be exploiting.)