In one of my campaigns a paladin recieved a quest from his lawfull good deity...
An angel came to him in a dream and told him a gnome alchemist had invented a substance that was both inexpensive and fairly easy to make, once you knew how, but also extremely dangerous. If the procedure were to get out to the public, it would mean that anybody would have access to 'fireball-level' destructive power at the cost of gold piece or two.
Naturally, the forces of law dreaded the anarchy and increase of crime this could very easily cause; so the paladin was to stop him with any means possible. Preferably with diplomacy and sound reasoning, but if the gnome refused to see reason the paladin would have to kill him. All notes, paper and alchemical substances in his small tower were to be destroyed of course...
The paladin had a lot of qualms about his quest: The alchemist was not an evil man, and he definitely didn't want to kill him, but he wasn't about to refuse a direct order from his God either! The angel had let him understand this was by no means the first time this invention had had to be stopped and it probably wouldn't be the last. As I said, the procedure wasn't very complicated...
Figuring the deity knew what he was doing, and probably would make sure diplomacy won the day, he set out to find the alchemists tower. If he couldn't convince the gnome to quit his work peacefully, then presumably a sacrifice had to be made for the greater good. Now, the paladin didn't think the murder of an essentially innocent man could be justified by any cause, but if it was God's will than he supposed it wasn't for him to argue.
The paladin and his friends found the gnome's tower easily enough, failed miserably at their attempted diplomacy and eventually killed him.
The party gathered up every scrap of paper and every single jar with even remotely alchemically-looking content and tossed it in a pile in the backgarden with some firewood, doused it with lampoil and set it on fire. Some of the jars exploded with a surprising amount of force...
This is as much gunpowder as I've ever allowed in any of my campaigns, and this was only after one player had been nagging me for a couple of months about why there wasn't any gunpowder in my homebrew.
"It's not a very complicated thing to make, really" he'd say. "Surely some alchemist somewhere must have figured it out..."
He was right of course, but I still didn't want it in my world, so I came up with this. Feel free to swipe it, it's good for an evenings sidetrack from any long-running campaign you might have going.
Edit: Spelling