The code does not leak out on itself. It takes people to take it. There are obviously some security measures you might use like traffic analysis and lock-down on the repositories so only authorized developers can connect to it.
But by the end of the day your best option is to make sure that no one WANTS to steal from you. Your team has to be happy, they have to be proud to work for your they have to be loyal to the company and to each other. If you have such team it's a simple question of explaining to everyone that the code has to be protected from outsiders. It will not stop a dedicated mole but will prevent accidents.
P.S. And yes, proper clauses in the contracts would not harm as well, at least they will make sure that the developers are AWARE that taking code outside is morally wrong.
