I see you set “Max recipients” to 0(zero) which restrict all new message sent to user.
If you want any role not to send new message (but admin can) then use role to role block to block that role.

Set “To Role” as “All Roles” and “Block For” as “New Message”.
If you set Admins in Front End PM PRO > Settings > Recipient > Can send to admin Then these feature is exempted. So users can send message using this feature if they are blocked by “Role to Role block” feature