Bandits are attackable NPCs that can be found in Pollnivneach, competing with the Menaphite gang. It's recommended to pickpocket them at Pollnivneach, because any bandit at Bandit Camp is often killed for combat experience. Pickpocketing a bandit at the Bandit Camp may result in the bandit getting killed by another player while you steal from it. Note, however, that bandits at the Bandit Camp only require 53 Thieving to pickpocket, while bandits in Pollnivneach require a Thieving level of 55.
When you have anything other than a blackjack equipped and attempt to knock out a bandit, your character says "I need a different weapon to do this. I want to knock them out, not kill them." This happens even if you use a rubber blackjack or aren't wielding anything at all.