The only times this feature gets used on our forum, it's been a previously banned user trolling. We'd like to just shut PMs down for guests. I can't find a setting that would allow PMs to be turned off for guests. Am I just missing it?
Guests technically are not allowed to send "PMs" as such.
Guests can send messages to the admins if the contact form is enabled for sure (it's a setting, I don't remember exactly where it is, sorry I have to check, but if you search contact in the admin panel you should find it).
Then likely if the guests are allowed to send "emails" to the users (it's a permission).
Apart from that I don't remember any other entry point for guests.
They are coming from your contact form and I would not disable it, it's the only way that someone can contact an admin without registering to your forum. I find it useful also for members that cannot login.
I made the mistake of enabling this feature. So far, the only people to really use it are people who decided to mail the admins with tech support questions instead of registering.
The tools is intended for legitimate uses, but it's nothing new that people misuse tools at any time.