If India would block Pakistan's water, Pakistan would have no choice but to wage a war. You cannot block rightful water of a Nation and expect nothing in return.
Now question is, what is India prepared to lose in return. By blocking water, India would receive international condemnation and any action by Pakistan would have more legitimacy than India's actions could have. Go on, try this action too and see if India looses more international backing.
Modi promise us that you would do something.. you are too much of a joke already.