Perhaps implement some form of market-based bonuses eg if a given games sells x copies, each member of the team gets a percentage of the retail price?

The answer is easy did the employee sign a contract if so take the legal means since the employer also signed the contract. If the agreement is worthless then find another means of contracting services, but never with a thief. The reason this makes sense is that no matter how gifted a thief they are still thieves. You cannot purchase honesty or negotiate its presence in another. It is the old joke if a women where asked the million dollar question and answered yes then it is established what service for a price might be had, further negotiations on price are only present to establish a bargain the service offering remains an established fact. So will the thief steal if they are a partner? The answer is always yes because they are thieves.