How to stop players from destroying your game

   Some nerd told me a story about how his players killed everyone in the tavern before the group even formed. This is disappointing that some players even act that way. No respect for the game or the master. Set up your expectations and rules before you start.
 
   The best way to avoid this game breaking stuff is to prevent it in the first place. Your evil druid wants to bash in in the skull of the bartender? Your edge-lord warlock wants to let loose a fireball in the weapons shop? The rogue wants to put an arrow in the queens face? As a DM you'll have to be the one to stop the action.

   Appeal to your players, or the offending player. Say something like, "You see johnny Mc-Edgelord about to cast a spell in the shop. Are you going to let him ruin your negotiations? What are you going to do about this? You know there are towns guards and the city watch right around the corner." Or appeal to the specific attacking player. "Are you sure you want to kill the barkeep? There isn't anywhere else to eat and sleep in this small town, if you make an attack, the entire town won't deal with you at all. You won't be able to buy weapons or rent horses." Prevent something before it happens.

   If talking doesn't sway the party right away, you can send in the troops. Don't even wait for the second round of initiative. Right after the player makes the first attack, the tavern guests run and get the guard. The guard shows up in the first turn to stop the battle. They're all level 20 and they immediately beat players to a pulp. If you don't want the players or player to be killed right away, they can be locked up. Knocked out. Charmed to be agreeable. Or anything else you can think of. This is your game and you wont tolerate the shenanigans. Make the barkeep level 20 and he can just knock the player out right away. Catch the players red-handed. Teach them that there are consequences to every action.

   If your game has lots of magic or technology, the shops and taverns will have magical wards, alarms, force fields, magic shields, etc.  The tavern has automatic detect poison spells set up. Or purification spells to protect the food. A player attacks the bartender with his staff? A magical force field stops the staff in mid swing and the player is hit with an entangle or hold person spell. In a lawless world, no one is surprised to see turrets in the ceiling of their fine dining experience. The bartender has a magical cup that is actually a port-key to teleport unruly guests out of his shop. It could lead a rude guest to a dungeon or the drunk tank lock up. Have anti-magic fields set up so mages can't mess things up, or zones of truth around the bar. Have magic mouth or message spells set up to alert the guards right away. Have a bard playing songs to control or sooth the crowd.

   Does your player want to set everything on fire? The city is enchanted with fireproof wood. The firemen use create water spells. Endure element spells. Control element spells. They can animate objects to make flying buckets of water to put out fires right away.

   The kings royal court will have high level mages or powerful generals who can take out a player in one hit. Take them out before initiative is rolled. The 5 guards against the wall have been holding their actions for days just in case some idiot player decides to attack the king.

   If the party or one player does get away with something, have an NPC witness the event. The guards or the nobles find out and send assassins or bounty hunters after the player. Put up wanted posters in every town and tavern. No one will work with these social outcasts. Have the players deal with being constantly chased. If they want to act like criminals, the world will treat them like criminals.

  Another problem my friend had was that the players hadn't even formed the group when the tavern was attacked. The players had no loyalty to each other and it turned to chaos. I prevent this problem by having a session zero or something like it. Start the game with the players already knowing each other and already agreeing to work together. It gets even better when the players have a pre-established common goal. This way the players will actually help and support each other without becoming murder hobos.

   Do the characters even want to travel with someone who is constantly attacking when they're trying to get weapons made or their loot being divided up. The cleric or paladin will not want to be associated with someone who just tried to commit regicide.  Will the characters tolerate being with someone who tried to assassinate the king? Have the group role play it out. The offending player will have to decide whether to change their character a bit or play a new one.

    If you don't want to give consequences in game, you'll have to be an adult and talk to your players. Talk to the offending player in private out of the game. Tell him you don't want everything to be destroyed. Tell him you have a story to tell and other players to work with. If the people playing aren't having fun then it's not worth it. Tell the player that it is a co-operative game and you guys have to work together. Tell the player they won't be invited back to the next game if they keep wanting to destroy everything. Don't invite him again. Put them on time out. Being a group game means that the players and the DM need to work together. If you can, try to establish these rules during session zero.

   If your player wants to continue destroying stuff, tell them to go play grand theft auto all by themselves.

Good luck and have fun!