What is the requirement for participation in discussions in order to send PM?
A new user gets this message
We are sorry, but you are not authorised to use this feature. You may have just registered here and may need to participate more in discussions to be able to use this feature.
What do you want to set it to? That is coming from the Newly registered users group. Do you want to use that group? If no, Set New member post limit to 0 in User registration settings, and remove all members from the Newly registered users group.
If so, change the Group permission for that group for Can send messages to multiple users and Can send messages to groups from Never to No.
I can stop all your spam. I can upgrade or update your Board. PM or email me. (Paid support)