'Sett blocking' typically involves blocking the entrances (there are usually more than one) to a badger sett and yes it is illegal. Terriermen from  hunts routinely and illegally block badger setts in the area where a hunt is planning to meet to stop a fox trying to escape a pack of hounds by hiding in the sett. It is a crime to do so. Setts might be 'soft blocked' with sticks or other soft material that badgers can sometimes push their way out or 'hard blocked' with heavy items or completely filled in and compacted. Badgers and their setts are protected by law and any blocked sett should be reported to the police immediately. Please mention that a hunt was (or had recently been) in the area.