Should fat people be prevented from buying junk food?

If somebody is over weight should they be prevented from buying junk food and attending fast food establishments , to help protect them from further damage to their bodies ? surely by getting fat they have displayed an inability to control their urge for food which is bad for them havent they ? If we face an obesity epidemic then why do we do so little to address the subject ? surely a new law which identified those with large bmi indexes through sight should be enforced for eg at supermarket checkouts and mcdonalds drive through s and kfc etc ,these traders have a responsibility to the health of their customers ? Do you think the obesity problem this country faces is one which is extremely serious and something which we cant carry on ignoring/