It does exactly as it says and I have to say I am impressed however I am adding some improvements:
A) It will know an outstanding invoice balance on a current user and won't allow access until it is paid in full. It will allow an invoice outstanding for 30 days before blocking the user. Once the outstanding is paid in full it will automatically unblock the users account and allow them access. It will display a link in the message to a payment area where users can pay to unlock and clear there balances only restricting other login area access.
B) It will have the ability to display the username of the user in the message.
C) Have a link to raise a support ticket so the user can flag issues with the admin on blocked status and allow contact to resolve.
Other than these improvements (which are just my personal desires) it is an all round to the point plugin and would like to the give the author a special thumbs up as it was just what the doctor ordered. Many thanks indeed.
I shall share any tutorials on my improvements in the support forum section of the plugin page as an when I have completed them.
Thanks again and keep up the good work!