Would it be possible to sign employees to a contract then? Flip this on it's head. i.e. we're happy to take you on board, but you have to work here for 2 years at X rate.
In the US (I know, this is a global community) we are largely 'at will' employment (a quick google suggests that only Montana is an exception). Such a contract is not enforceable. To lock people in we use things like vesting - you get stock grants, bonuses, or what have you, but only vest into them over time.
