I understand how this can be confusing, but I think from reading this you are after a different new setting that will handle this use case. It would be called something like disallow_personal_message_groups, and any users in the selected groups will not be able to receive PMs from anyone but staff (moderators and admins).
The personal_message_enabled_groups setting is not intended to be used as a block list. It’s a bit of a conundrum; maybe we could change the existing behaviour so people who are not in personal_message_enabled_groups cannot be messaged by anyone but staff either? I am not sure how many expectations this would break.
I will leave @tobiaseigen to weigh in as well.