Steve it takes two to tango. If you wanted to keep your female koi from spawning you could send them to an all girl Catholic pond.
You could probably keep them away from males until they were a certain age easy enough. If it was better to wait until they were 4 instead of 3 I think it could be done, and that's what I'm trying to get at. How ever if any body has any "my female begin popping fry at 2 and grew to a 100cm" stories, feel free to post them. If the eggs are developed during the winter when growth is not taking place I don't quite understand how this would be competing with growth energy. Now if a female spawned early in the season, and then spent energy in the growth season making more eggs it might make sense. Beware of the double clutch I guess.
Mitch