Thank you @osioke, that’s indeed an option. I didn’t know a group could “control” the trust level - I mean, adding or removing from group would add or remove the trust level. I thought it was specific and scoped to the user only.
Another alternative would be allowing personal messages only for people we choose or send messages to. Is it possible? I mean, disable personal messages for everyone (except for staff of course) and then manually allowing specific people that we would allow to send us a personal message (it could even be temporarily).