How to handle prepaying suppliers?

We have a supplier who operates on a pre-paid basis similar to a pay-as-you-go phone schemes.

We maintain a positive balance with the suppler. Each time they issue an invoice they will automatically take the money for that invoice from our balance. When our balance gets close to zero we will top-up our balance by paying them some money.

What's the best way to handle this sort of arrangement in Clearbooks? When I use the bank statement import tool or "Manage money" I cannot see anyway to pay money to a supplier without an associated bill. If I don't have an unpaid bill to allocate the payment to then the system automatically generates a bill for me which is not what I want.