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That s not feasible, not only bots but entire applications are built over the blockchain. A captcha wouldn't let these application posting. Or every application would need to implement a captcha on their side but there is no way to force them!
Welcome to Steemit btw, I am glad to see that you are new here and already trying to improve the system. I am getting you to 30 rep! (EDIT: 31 actually)