You can limit the payment methods on a proposal before it gets sent out, so that when the client clicks to pay, only the payment methods you have enabled for that booking will be offered to the client at checkout.
Start by clicking the Manage Booking button on the proposal or booking.
Select Edit Proposal Settings
In the Payment Methods box, choose Limited Payment Methods
Un-check the payment methods that you do not want to offer
Click Save to lock in your payment methods on this booking
Short Video on Payment Settings