With the slave contract system, they'd just offer them fights and keep extending their contracts indefinitely. Starve them out. They can't go do anything else and are totally locked down. Slowly one by one they'd start coming back to feed their families.
Whenever a fighter they want is getting in a good position, they are all on top of it with every dirty tactic they can legally throw at them so they can lock them down again. At no time are there too many fighters in a good position to make a move like this. UFC has them by the balls and both sides know it until some corrupt politicians get greased the right price to even the playing field.