If your software is desired enough and done well people will pay for it. You are always going to have a % that never will. Rather than wasting time trying to stop them, focus on improving the product.
99.9% of companies don't want to steal something and rather not change your software for there needs. They rather buy something out of the box that will work.
Keep in mind Fog Bugz from Fog Creek ships with its complete source code, and they have no problems selling there software because it is a quality product.
Quality is important, focus on that not security.
