I've seen people try that, but the admins just sit there quietly watching them loop for an hour or something, then note how long it takes them to respond to a simple hello.

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.