The Durand Line is 2640 km. To completely seal off the border, you'd need a post every 500m at least. That means over 5000 posts, each manned by a section of 10 men at least. That translates to 50,000 troops! Where are you going to get them from? Even if 50% of this is to be manned due to terrain configuration and likely infiltration routes, you'll still need 25,000 troops or two divisions.
Now add the problem of logistics. How are they going to be supplied when there is no road communication network in most areas? Air dropped supplies? Or chopper support? Do you know the costs involved for such an operation?
It's easy to shoot off ideas but their implementation is impractical.