Actually, I find this kind of weird since here in Brazil copyright law is only enforced in the business world. Piracy rates are wild, but any medium-sized company has original software.

So the point is that, every company that's big enough to need an ERP software is big enough to worry about software piracy. Not to mention that contracts are enforced (the problem might be that you're not able to get them to sign a contract with you).

I know I'm not answering your specific question, but I think this might be slightly more relevant. As tirwhan wrote, the problem lies in your business model. As I understand you expect to provide them with a product, provide no support whatsoever, and still get a constant revenue stream.

Things just don't work this way.
You can sell them your product and then they may choose to purchase future upgrades from you or hire another company of their choice in order to further develop the system, if they're not pleased with your services anymore.
The other way around is that, instead of selling the product, you sell the service. So you'll have a revenue stream provided you keep doing your job, which is to maintain the software.

Both of these models work pretty well. Anything else probably doesn't.