The level of interaction between players on Discworld is controlled by the status of 'player killer'. By default players are not player killers which means that other players cannot kill them, steal from them, loot their corpse or place contracts on them at the Assassins Guild.
Players who wish to do these things must apply to be player killers using the 'apply' command. Once registered they will be able to do these things to (and have these things done to them by) other player killers.
Once you have decided to be a player killer, there is no turning back. To emphasise this again, there is no turning back ever.