It is possible for payers to make further bookings if their account is in debt providing the 'Pay later' payment option has been selected for the club.
You can see this by clicking Amend payer settings within the club
Then, if you wish to, you can amend the payment option to 'Pay now' to limit people booking if they have a debt.
If the Club is set up with the 'Pay now' payment option, any outstanding debt will be automatically added to the basket when the payer selects to make a booking for that Club. Payment will have to be made before a further booking can be confirmed.