Make a 3x3x3 hole and a 1x3 pillar of obsidian with a pressure plate on top and trapdoors around the hole and run redefine down the obsidian connecting to every trap door this works best on a server were you can own a block but it's obvious with the pressure plate

Or you could place obsidian over a chest with TNT behind it they will get bored and not pay attention accidentally hitting the TNT and causing a explosion