Do you mean Ragwort?

The only way that I know is to pull it up, plant by plant. On a visit to a local estate on Wednesday we noticed a 'summer student' systematically tackling a whole 10 acre field of ragwort. He had a tool rather like a garden fork but with only two prongs. I had not seen one of these before.

Ragwort used to be a notifiable weed (you had to report it to the Min of Ag.) as it is poisonous to grazing animals. All I can remember is people pulling it by hand as Bulbaholic suggests, which seemed to give adequate control.
The info below comes from a website about its control...

"About ragwort:
Ragwort is highly toxic to horses, cattle and sheep, causing progressive and irreversible liver damage. By the time the symptoms show, there is no remedy and the animal dies or must be humanely destroyed.
Ragwort is highly prolific; it propagates by blown seed and a single plant may produce 150,000 seeds, with a 70% germination rate.
Ragwort is hard to eradicate; seeds can survive 20 years in the soil before germinating and any root left behind when it is dug up will re-grow.

Where to get help: The following booklets are available from Defra publications on 08459 556000: PB 4192 - Identification of injurious weeds; PB 9840 - Code of practice on how to prevent the spread of Ragwort.

CLA: The Country Land and Business Association is the premier organisation safeguarding the interests of those responsible for land, property and business throughout rural England and Wales. "

Host to the beautiful Cinnabar moth, of stipy black and yellow caterpillar. They conatin cyanin, virtue of the plant.