Well they can always choose to cancel their contract with Tesco and find another buyer for their meat.
Reading the article it sems that the buyers were "disturbed" by the lack of control over the dogs when visiting the farm and the effect on the "terrified" sheep. If that means the qualithy of their meat is going to be affected and thus Tesco's image, they have every right to make the request, or find an altenative supplier.