Get a Hexblade to turn them into a specter. Kill the specter. The MM seems clear on the subject and it’s a fun alternate use of the 6th LV Hexblade ability.
The Specter's lore entry (p. 109, MM) says:
When a ghost's unfinished business is completed, it can rest at last. No such rest or redemption awaits a specter. It is doomed to the Material Plane, its only end the oblivion that comes with the destruction of its soul.
