Although Filepass is built around the ability to let clients stream files anytime, but not download them until the project balance is paid, we occasionally get requests for a feature to disable file streaming until the client has paid.
There's already a way to do this, though it's only a workaround.
By uploading a .zip archive, the client will not be able to hear the files until they have downloaded the .zip and unpacked it. Then they can listen to the files using their media player.
To learn how to upload a file, see this article.