You can probably cover some of it (reverse engineering) in your license. If it's really that simple that anyone could re-build it that easily chances are you don't have something that unique.
If it is unique it probably has enough value to a business to simply use it to make money. Most businesses focus on making dollars with the right tools, not saving pennies and spending thousands of dollars of their own to re-invent the wheel.
When I make something and they get the code, I break it up into two parts.
Type "A" is stuff they requested for their competitive advantage. Custom.
Type "B" stuff is my pre-made libraries and code base that I re-use. They get a license to use, but not re-sell.
If they were using my product altogether, I'd make it so that they simply own a license. I'm sure there's a ton of people who can weigh in on this.. good question though. I'm not entirely sure if there's a lot to worry about.
If it was a million dollar sale you would have hired a good lawyer by now. Get something built and sell it! :)