Obviously the Watchtower could end their shunning policies if they wished, just by issuing some "new light." From a legal standpoint, however, I'm not quite sure how you would be able to enforce any action against shunning. Would you require that individual Witnesses now spend time with people who have left the organization, whether they want to or not? How could you ever enforce such a requirement? What if they simply don't like the person, apart from any religious considerations?
Would you require that the Watchtower issue a statement repudiating the doctrine? What if they did? Couldn't shunning still be carried out as a "private conscience matter" (as, in fact, they would like you to think it is done now)? Wouldn't this set a dangerous precedent of government dictating what doctrines a religious group may and may not teach? Actually, I can see where interference by government of that sort could easily be a back door to the establishment of a state church.
The topic of government agencies somehow pressuring or forcing the Watchtower to "end shunning" comes up fairly often, but I can't imagine how the logistics of such a move would work, apart from some serious erosions of religious freedom that would negatively affect people of all religions.