Dumping spot for human and animal corpses. Each grave can be set to individually accept or deny strangers bodies, colonists bodies, and animal corpses.
Putting colonist bodies in graves instead of dumping zones will prevent the "saw colonist body in dump zone" unhappy thought.
Alternative solution to using graves
Define a Dumping zone somewhere (preferably under some kind of roof), set the dumping zone to only accept bodies, when there are bodies in the dumping zone, draft a colonist and equip him with a Molotov cocktail (or a frag grenade) and throw it at the bodies. If one body catches fire it will spread and incinerate the others (the roof prevents the rain from extinguishing the fire).
Using a butcher table add a bill to butcher the corpses. This will not only get rid of them but also provides you with meat and leather. Be careful, butchering human corpses will grant the " Butchered humankind -6" debuff for the cook and the "We Butchered humankind -6" debuff for everyone in the colony. Note: this only works with non-rotten corpses. Note: colonists with the Psychopath, Bloodlust, or Cannibal trait are unaffected by the debuff.
Before 0.6.532 There is no easy method of permanent corpse removal from the game. Using explosive charges (research option) placed in the center of graves allows for the removal (requires 2 charge detonations for corpses to reach 0 health and be removed from the game) of the surrounding corpses inside the graves. The grave structure is not harmed and can be reused indefinitely.