Several questions/items:
1. If you are "signing them up on the site" and they aren't paying, are they continuing to use the site? If so, just suspend their account until they pay or if they don't pay after that close their account.
2. Is it a product that you are getting signed contracts with an agreement to pay? If so there is always the possibility of that you could pursue a legal angle.
3. I would focus on finding out what their hold ups are on paying by digging deeper. Offer to hold their hand on the payment process. From what it sounds like these customers potentially like your product and may see an initial use in it, but it is not something they are willing to pay for and are trying to be nice and hope you go away by just not paying.
4. In the future you could raise the price of your product and offer then offer various discounts for paying on-time or early.
5. Is it a one-time fee or subscription service? You may try playing with the pricing and payment terms (i.e. if you only offer a monthly price try adding additional terms like quarterly or annually. Or if you only offer a one-time fee try breaking it up over time).
(Edited to fix typos)