Adult ball pythons going off of food is of no concern at all, it's perfectly normal. They can fast for six months or more, the longest I had one fast was about eight months. I have yet to hear of a ball python passing away from starvation, I've even seen people feed hatchlings very little to where they hardly even grow at all in the first year, not that I support that kind of care but it seems some big breeders to it to save costs, the main point being that it won't kill the snake (probably not too good for it though...). Right now I have several that are fasting out of about 50 snakes. It is winter and I'm in the middle of the breeding season but it seems that no matter what time of year some go on a fast. I also have a couple females now that are HUGE with eggs and not eating. I'm guessing they won't eat until after they lay. I also have a couple that refuse to eat anything - frozen thawed, pre killed, or live (rats or mice).
Frozen thawed is a bit more tricky to feed than live, I had about half of my snakes that wouldn't eat using frozen thawed or fresh killed. When I switched over to live I had almost all of them eat. Feeding live can be dangerous for the snake and I've had a few battle wounds, it's best to feed rodents on the small end of the range when feeding live to give the snake the advantage. For rodents larger than about 80 grams I feed pre killed with CO2. In fact if you have an aggressive eater it's best to euthanize with gas just before feeding.
I never waste a single rodent during feeding. When I feed live I leave the rodent in the tub for five minutes. If it's not eaten within that time I'll take it out and move it to another tub to give a different snake a second feeding back to back. I have enough snakes to use up all the non eaten rodents. If you feed a pre killed with CO2 and they refuse you can always freeze it, then use it again as a frozen thawed the next time around. If you thaw a frozen thawed I would never refreeze it. If you have multiple snakes it's always best to try to feed the picky ones first then follow up with the more aggressive feeders. Some people feed half of their snakes every 4 days then feed the other half 3-4 days later. That way you have extra snakes per feeding to use up any uneaten rodents.
Another option is to get a second snake that is a more aggressive feeder. When I started out I used frozen thawed quite a bit and bought a dwarf reticulated python as my garbage disposal. She would eat anything that was left over from all the other snakes. Actually a pure super dwarf retic would be the best, very aggressive feeder and won't get too big. I've also seen people use other disposal creatures like alligators or monitor lizards that can eat an endless supply of left over rodents.