Regular topping. Livestock will then eat some of the younger stems and leaves, so just keep topping so it doesn’t set seed. You never fully eradicate it but it can be controlled.
Once you’ve really knocked it back, sheep will hold it back better than other species. And I think primitive breeds eat it better than commercials.
Weed killers only keep it at bay for a year or two too.
The old way was to “spud” the plants. A tool like an apple corer stuck on the end of a walking stick. As you check the stock, dig up the dock plants by the roots using the spudder. Because docks are extremely deep rooted, they will, again, make it back in a year or two.