While you have the game mechanics to starve a realm, no realm would let you get away with it. As soon as you started to do it, c you would get kicked out. Or the lord would feed the region himself. It's real easy to say you'd do it. It's a lot harder to actually do it, and get the support for it.
It's most useful just before a predicated rebellion or banning of a lord to prevent secession.
Remove all food from their regions, drop a ban from the judge, and remove all militia from the general. You're set up for an easy civil war.
