Punishments

unishment is the final effort of enforcing the rule of "Justice" in the One Piece world. The One Piece world has various punishments for crimes. Criminals who gain a bounty can risk bounty hunters chasing them, or in more serious cases Marines. Those in the grand line also can risk being taken by kidnapping groups and sold off. Marines who capture a criminal may hold onto them until they are taken to a more suitable prison, like Impel Down. Some will spend a brief moment here, but most will spend the remainder of their lives here, even dying in the extreme nature of Impel Down's levels while the worst criminals are placed on the very low levels of Impel Down essentially they end up being "erased" from history. For those who are deemed too much, they are publicly executed and made an example of in order to deter others from doing activities similar to what they did.
The most popular method of execution seen in the One Piece world was used on Gol D. Roger, Montblanc Noland, and Portgas D. Ace. It involves placing a criminal on a high execution platform, above the heights of most individuals, usually in a high populated area where the normal citizens can see the criminal be punished. From here everyone can watch as two executioners decapicate the criminal with a long blade.
Another, less spectacular, method is shooting by a firing squad. It seems to be used on less prominent criminals like Nugire Yainu who was executed under the name of Captain Kuro.