You can block a customer's membership from being used if they have overdue pending offline payments, i.e. cash, cheque.

Firstly, you'll need to decide what is classified as an overdue payment.

This setting can be found under Settings >> Registration.

Enter your desired number of days before a pending membership payment is considered overdue.

Now any payments that are considered overdue, will be marked so from Billing/Revenue >> Pending.

Click on 'Options' and you will see the option to block registrations for this particular membership.

The customer will not be able to use this membership until the payment has been confirmed by the business.

The block is automatically removed when the payment is confirmed.

You also have the ability to manually remove the block.

Click on the customer name to take you to the membership.

Click 'More Information' and you'll see the option to remove the block.