Do you hate it when necromancers and vampires resurrect an enemy you just killed no longer than five seconds ago? Do you wish there's a way to make sure they STAY dead? Well just ask your friendly necromancy user and they'll say that not even Dead Thrall and the Ritual Stone can bring back enemies that were either turned to ash or beheaded. And each of those conditions can be done in these simple - though maybe time consuming - ways.
This can be done with just one perk and it will depend on whether you use a one-hand or two-handed weapon; Savage Strike for one-handed, and Devastating Blow for two-handed. These perks work with any melee weapons, even blunt weapons like maces and warhammers. Note that these perks only work on humanoid enemies - men, elves, beastfolk. Draugrs are not affected, though the killing animation is still used. While the decapitation chance is unspecified, both do give a 25% additional damage to standing power attacks.
Ashes to ashes
This can be done with the Disintegrate perk in the Destruction skill tree, the master Destruction spell Lightning Storm, and the Dragonborn DLC ability, Dragonborn Force.
Disintegrate requires 70 in Destruction and proves also useful finishing off dragons. And, yes, you can still get their souls.
Lightning Storm requires a maximum 100 in Destruction. Its use costs 138 magicka per second, but the Master Destruction perks reduces it to 69. Taking the Necromage perk makes it especially useful against the undead.
Dragonborn Force is gained from the Black Book in the Dragonorn DLC quest, The Gardener of Men. It increases Unrelenting Force’s environmental damage and helps kill weaker enemies when dealing with groups.
The Wabbajack does both in addition to transforming into small animals and items like rabbits and sweetrolls, though these are 4 of the 28 random effects.
^_^ Have fun making people stay dead.