You can use the mycred_userpro_insufficient filter to adjust the message the user sees if they run out of points and can not send messages.
If you want to reward your users with points for sending / receiving messages, make sure you set a point value other than zero to enable the hook and make sure you have provided a log entry template.
If on the other hand you want to charge your users, make sure you set a negative value for sending messages. Make sure you also tick the “Prohibit users with zero points from sending messages.” checkbox! If you do not tick this checkbox, a user will be able to continue to submit new messages and gain a negative balance!
If a user can not afford to send further messages, they will see an error message in the chat window and the “Send Message” buttons will be hidden.
||Give or take points from users for sending or receiving a new message.|