Are you sure they're guppies? If you're in the US, you're likely seeing mosquitofish (Gambusia affinis or G. holbrooki). It makes no difference to training them onto prepared foods, but could make a difference in getting them to breed.
I suggest you just keep offering prepared foods. Starve them for a day or two if necessary; it won't hurt them. Fish have preferred foods just as people do, and may not want to eat a new item if more familiar food is available.