I don't think I would cut back the leaves very much since that's where photosynthesis occurs and that's how plants get energy. You could maybe pinch out the growing tip of the vines so you wouldn't have a lot more new growth. Now keep in mind I grow yellow squash not zucchini ( never been a big fan of zucchini), but it seems to me that once they start fruiting heavily my squash usually slows way down in the green vegetative growth dept. Cutting back on fertilizer (like miracle grow) will also help since the nitrogen encourages new growth. Just my opinion - I don't want to offend any seasoned zucchini growers.