how to prevent people to sign up without paying for subsite

I am working on my multisite above. I want to sell subsite subscriptions. I am doing this with the "new blog templates" plugin. However, the problem is when people go to the sign up page:, they can sign up for a site without paying. I don't want to use the prosite plugin to collect payment. I have woocommerce installed with paypal standard working properly. I have been told by some developers that I need to write code in the functions.php folder to verify that the billing address that a user uses to sign up is already in the system. They said I need hooks and filters in the functions.php folder. However, none of the developers I have hire have given me a working code. This means that anyone can sign up for a new subsite without paying. Can you please let me know the code for this?