This is a mad, twisted idea, only for dealing with mad, twisted people. (That was a warning.)
Let the munchkin do what he wants. He kills someone, come the guards. He kills the guards, comes the army. He kills the army (one-by-one or all at once, depends on the mood), come other folks, more powerfull. He kills anyone.
...time passes, blood flows in rivers.
Comes THE God, head of the local pantheon, WTF is he doing, killing everyone? I think we know what happens, after a hard fight, is the chief deity killed. Proclaim him Master of this World, now that he killed the former one. Give him a few additional destructive abilities.
Now the catch: (finally)
As he is bound to this world, like the former Master was, he cannot leave. Never. I mean never ever can he leave this blasted world. He did NOT take any creative abilities, if I guess rightly?
And the more 'fun' he has before he notices it, the more destroyed and empty is the world he rules. Used his powers too much?
Now you float in the emptiness. Around you slowly rotate the few rocks, all that remains from the world of .... .
And he cannot leave, never. He is immortal, too, by the way.
Should he ever succeed to destroy this 'dimension' or 'plane' (the place the former god created to contain the world), it means he dies with it , forever and absolutely dead.
May be a solution.