Follow these guidelines and it shouldn't matter if the contents of your entire source code repository is posted all over stackoverflow:
http://geocities.com/mdetting/unmaintainable.html
Oh, and show your developers that you don't trust them by blocking access to parts of the source code, scanning outgoing/incoming email etc. That is a surefire way to make them want to stay around... ...nothing improves morale like a bit of mistrust in the workplace.
Another cool way is to tell one half that they are "team a" and name the other half as the untrustworthy "team b". Then reverse it and say the same thing to the "team b" members. Encourage them to keep an eye on the "bad guys" in the other team and to report any signs of illoyalty to you. Sprinkle a few "conflict inducers" (e.g. tell "Joe": 'do you know what Ed says about you behind your back?') etc. Works wonders if you set up the developers against each other and create a few [invented-by-you] conflicts here and there...
(Eh, and no, I don't actually recommend any of the above. Just kidding. But I have seen people use all of the tactics above. And it didn't work.)
