True Cremation

You invoke flames into inanimate corpses you can see within range. The magical flames burn and melt the entire body until it's nothing but ash. The body is reduced to a skeleton in 1 minute, and in 10 minutes, the skeleton is reduced entirely to ash, including teeth, preventing them from being raised into undead by most means. 

The flames cannot spread elsewhere, cannot deal damage to other creatures, release no smoke or odor, and only when there is nothing but ash do they extinguish. This spells casting time can be reduced to 1 action if you expend a spell slot of any level.