Just tie up something that looks like pelleted fish food. If you really want to get them, throw some real pellets in the water before you throw them the fly. 🙂
Koi are pretty omnivorous. If they were domestic, they’ll be used to pellet food. If they’ve been ‘wild’ for long, they’ll feed on whatever is common in the pond.
I keep tropical fish, and it really pisses me off when people release fish that get too big for their tanks. The environment suffers because of their bad planning.