How to block a customer

The system doesn't have the possibility to block a customer. The reason why is because the customers are identified by their phone numbers in our system. In such a case, you'd be able to block the phone number, but the customer can use a different number to make a new reservation, which means that you wouldn't even know that it's the same customer.
What we do suggest?
Please add a "client note". Once the customer will try to make a new reservation, you'll be aware that's the same customer and you will have an option to cancel his reservation. The customer will also don't know the reason, therefore he will not try to cheat the system by using another phone number.
 So this way will make a win-win situation: you will know all statistics and customers don’t feel bad to decide to start cheating with other phone numbers or etc.