How about a simple code when someone PMs you, you are asked wether you want to accept their PM or Not? You get a notice of what they pmed you (the first line) and if you want to continue to talk to them /query
What I have is, when someone PMs me they automatically go under my PM Ignores unless they are in my PM exceptions. So if some random person PMed me they'd be ignored right away, I have to either PM them to start a conversation or put them in my exceptions.