As everyone has mentioned , it all starts with your NDA and your employment agreement. Next keeping a highly motivated team that has a healthy culture of ethics. Finally from an architecture perspective you might be well advised to figure out a way to modularize things so not all people get all source. There are ways to do this which can be a benefit to the system you are making, the teams and responsibilities of the staff as well as a physical separation of concerns and access to help mitigate your problem of code availability.

Good luck!