Buy potions from the priestess wandering about town which will remove one point of CON loss per potion, or just stay alive for a level or two. It gradually goes away as you get experience.
You also be take other nasties from sting, like low experience gain. To get rid of these, keep drinking the potions until the EXPERIENCE command shows you have no Death's Sting or wait it out in the same way as CON loss. Potions always fix CON first and only move to removing the other death's sting effects once CON is done.
