It can be done easily. You can follow https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/only-admin/ and https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/role-to-role-block/

Summaries:
1. Set 1 (or more) in Front End PM PRO > Settings > Recipient > Max Recipient
2. Go to Dashboard > Front End PM PRO > Settings > Security > Role to Role Block. Click “Add More” and select “From Role” to “Subscriber”, “To Role” to “Subscriber” and “Block for” to “New Messaage”. Then save changes. Now no subscriber role users will be able to message to another subscriber role users.

Let me know if you have any more question.