I have used side cutters to snap the teeth off rats when confronted with rescues that only feed on live. Further to that, they usually bang their heads on the door frame of the cage on their way in. Once the snake discovers eating injured, then pre-killed food, it is only a matter of time before they accept thawed.
Another trick I have used is to take the water away for at least 48 hours, and then offer a wet thawed prey item. Often the snake gets excited while trying to "drink" off the prey, that they will open up and swallow it.
For the price of the snake, and the resultant vet bill, you can buy a lot of frozen food.