They probably could've just covered it in something so that it doesn't have enough oxygen to ignite and resurrect.
But ye, I always wondered why don't characters in other works of fiction just feed the immortal enemies to dogs or such.
Dunno why you guys say that it's bad now, it was always like this.
And knowing this manga I'm pretty sure that this cliffhanger is just a spoof.
But even then, Kyuu's still better than Haru.