Hi
My experience with docks is to keep cutting them down close to the ground. Eventually they give up and die off as it keeps weakening them. Make sure you burn the plants you cut down as you don't want to seeds to spread everywhere. It takes a long time but I have won the battle in the end. We dealt with 5 acres of ground in this way. My two, now grown up, children still have nightmares of summer days spent 'dockin choppin'.
Jackie