How do you prevent hired developers from stealing code?
You may find that you think your source code is the valuable part of your business, but you can always build that again. Your real advantage over your competitors is usually in the people you hire, and in the business relationships that you establish in the course of naturally doing business.
Don't hire people you can't trust.
Break the app into sections and only let people work on a subset of the app, never getting access to the whole thing.
Make it worth their while - you're opening a company, hire people and give them some stock options. Make sure it's more attractive for them to make you succeed than otherwise.
How about keeping them all happy and show that you appreciate their work?