Is it possible to punch someone's head off?

I thought it was a bit of a shame that Skyrim removed the Hand-to-Hand tree that was in Oblivion. But I was curious if there was a way to inflict massive damage using hand-to-hand attacks.

According to a friend (who may well be unreliable), it is possible to decapitate enemies using a punch alone.

My question is... is it?

No. The only way to remove heads is via a finishing move animation. And the finishing move animation using only unarmed is the neck grab and neck snap, which does not remove the head. You'll have to use a sword for a decapitation.

However, there are mods that allow you to perform unarmed decapitations.

No, it isn't possible for a simple reason: to decapitate enemies you need Savage Strike (a skill that require a one-handed weapon) or Devastating Blow (a skill that require a two-handed weapon). There is no skill that give a chance to decapitate your enemies with unarmed attacks.