As of Minecraft 1.8, you can kill a wither boss with a relatively simple command: /kill @e[type=WitherBoss]. This will literally do what it says on the tin, and kill all wither bosses in the world (hopefully just one).

If you're not in 1.8, or don't think you can type that with the proper capitalization in time, you can also set the difficulty to peaceful with /difficulty 0. If they had any pet creepers or anything like that, you can spawn them back in and give them name tags again using spawn eggs, since they will also go away upon setting the difficulty to peaceful.

As of Minecraft 1.11, you can kill a wither boss with a relatively simple command: /kill @e[type=wither] (or /kill @e[type=WitherBoss] between 1.8 and 1.10). This will literally do what it says on the tin, and kill all wither bosses in the world (hopefully just one).

If you're not in a version after 1.8, you can also set the difficulty to peaceful with /difficulty 0. If they had any pet creepers or anything like that, you can spawn them back in and give them name tags again using spawn eggs, since they will also go away upon setting the difficulty to peaceful.

The easiest thing to do here would be to set the difficulty to peaceful with /difficulty 0. If they had any pet creepers or anything like that, you can spawn them back in and give them name tags again using spawn eggs, since they will also go away upon setting the difficulty to peaceful.

As of Minecraft 1.8, you can kill a wither boss with a relatively simple command: /kill @e[type=WitherBoss]. This will literally do what it says on the tin, and kill all wither bosses in the world (hopefully just one).

If you're not in 1.8, or don't think you can type that with the proper capitalization in time, you can also set the difficulty to peaceful with /difficulty 0. If they had any pet creepers or anything like that, you can spawn them back in and give them name tags again using spawn eggs, since they will also go away upon setting the difficulty to peaceful.