Require in the contract that you are not their only client, but lawyer it well. "We only work with contractors with a diverse set of clients as we believe that diversity promotes excellence."

Create buffer companies for those contractors to also work for indirectly for you if necessary. You might still have to pay some taxes, but you won't have the same problem as the Swiss salary cap is trying to prevent if it were implemented.

There will always be ways to exploit systems. You see it as exploitation, but those running the companies see it as necessary to compete.