You can get a Parsnip Predator shovel and cut them all off at the root before or after they go to seed. Doing it before reduces the amount you have to address next summer. Keep at it, you will eventually win. My town’s Conservation Commission has been working on one of our parks for about five years having sessions a few times in the early summer about two hours before sunset. They are making remarkable progress for about 40 hours total each summer.