I'm going to make up a new term: Golden Rule Chicken
The Golden Rule: Do unto others as you would have them do unto you. In RPGs and LARPs, the rule can be applied to character power: If you can do it, so can I. Of course, generally the GM employs things the players cannot -- deus ex machina needs to be wielded by a qualified deo, right? But the GM is also limited in what he can do to the players. It's not a shared story telling experience if the GM randomly kills them, so land mines, accidents, and poison all need to be turned into opportunities for dramatic action, not just instant, unexpected death for the players' characters.
Chicken is a game where two people race cars toward each other, and the chicken is the one who steers out of the way first. It's a stupid game. Don't play it.
Golden Rule Chicken is when an RPG player plays chicken with the golden rule. He wants a quick, easy, and certain-to-work way to kill bad guys, capture people, get away with anything... but he doesn't want it used against him. Either the GM has to be the chicken and let him have power that the GM cannot employ for the story, or the player has to be the chicken and concede that he should limit himself for the sake of the game, the story, or the genre, or verisimilitude. It's a stupid game. Don't play it.
What do you do if a player tries to play Golden Rule Chicken with you?
Option 1: Say no. Authority is as authority does, and the GM has the authority to veto something that makes the game less fun.
Option 2: Take out the nerf bat. Make the poison/car bomb/etc. fairly weak. When the villain who should have died survives, horribly disfigured, and wreaks his revenge, it will be pretty dramatic. Poison is ripe for nerfing because in real life, poison takes minutes to kill you. And in those minutes, you can shoot a lot of bullets, cast a lot of spells, etc. Plus, monsters tend to be immune or resistant.
Option 3: Consequences. In D&D, using poison is an evil act. In the World of Darkness, hurling sacks of dynamite around gets Homeland Security coming after you. Warn the players of the consequences ahead of time, and don't let them try to argue their way out of it. Explosives are ripe for consequences, regardless of the genre. Space marines could be court marshaled, steampunk heroes could fall on the wrong side of Scotland Yard, and vampire hunters could get the FBI and ATF looking into them.
The thing is, you can't just let them have it. I know it sounds harsh and arrogant, but I've run games where I let the players have an instant death attack. Things got really boring after that. You have to take it away or contrive something rather silly like "the villains gave all their henchmen antivenom kits just in case they meet you." Or "Grenades won't work. They brought Bruno Mars."
If you think the super-weapon would be genre appropriate, and the consequences would also be fun to explore (using a truck bomb to kill a vampire in Hunter), employ Option 3. If you think the super-weapon would be genre appropriate for the game you're running, but not especially interesting and dramatic (using bombs in Godlike, for instance), use Option 2. Else, use Option 1.