You can deny access to a customer / customer account by using a ban. The following steps must be taken:
1. Click on the customer and on the customer line.
2. Click on on "..." button at the top right. Choose the option: 
3. Verify on the customer detail page whether the account has been deactivated.
4. Obviously you can undo this but you have to choose the menu option: (Give access to MyParkingPro).