They won't be able to access the app. The client will be asked to contact their coach when they try to log in.
For the client to regain access, they need to log in to Kahunas website, they will be prompted to make a payment.
If you need to allow the client to access the app till they make the payment, you can click "actions" in front of their name, then click edit, and switch the client to "No, they don't need to pay", once the client is ready to pay, you can switch them back.