One of the best tricks is to show them messages via a method outside of normal chat which a normal player would see on their screen, but which a bot would not receive as 'chat'.
As for not talking to anyone, a surprising amount of people play MMOs just like that, so it's not really atypical for a player to never communicate. Runescape even has an account choice, "Ironman Mode", where you have to play the game self-sufficiently, and can't trade with or rely on any other players. You can still chat with other players if you want, but you don't have to.
Or in some games, they can send messages in a way that a human would see, but not a bot who expects the messages to come over chat. For example, waving a sign in front of the character's face with a message or whatever. It helps that the admins can also hide from normal presence detection, even though they're visible on the bot's screen, visually.
I've literally watched admins ban a bot using these precise countermeasures. The trick is to always keep giving them new things they haven't thought of to adapt to.
