Certainly if Coke and Pepsi, Apple and Microsoft, can have these types of agreements with their employees, I wouldn't see why a baker couldn't have the same thing. Why don't you contact some better known bakers and ask them the same thing. Like, say for instance Pink Cake Box. They seem to be quite open about most things (like putting up a video each time they create a new "masterpiece" cake) Surely, if you were to ask them a benign question like, "How do you keep your baker's from stealing your recipes?" They would be happy to answer the question and maybe even send you a copy of their Non-Disclosure Agreement that they have their staff sign.
Another way, I think that could help you is to do like most of the larger companies do and assign only one or two (depending on your volume) people to be responsible for only the baking in the business-no decorating, unless it's dirty icing, or making gumpaste flowers or figures in their down time. This is-at least how I understand it- the way the it is done at Carlo's Bakery (Cake Boss), Ace of Cakes, Ron Ben Israel, and Sylvia Weinstock's bakery's.
Anyway, HTH...