[user104658]

Surely the simplest solution would be to write it into your employment contracts that this isn't allowed? Whether or not you can force him to sign a new contract is another matter, though. The company I work for tends to, basically, make things as difficult as possible for people until they cave and either sign a new contract or quit. Or they use bribes - a "bonus" in the next pay if you sign whatever it is they happen to want you to sign.

Otherwise I think it depends whether or not you could reasonably justify this as theft. He could claim that he didn't consider it such, but then, if you tell him (in writing) that you will consider it to be theft in future then he doesn't really have any comeback next time.