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Adding Captcha checks is a quick and easy way to keep bots from spamming the platform. For a website it is easy, to add it to the blockchain would be more difficult. (but not impossible)
Steemit could easily allow users to filter out bots by simply adding personal user blocklists.

Users blocklist would be a really powerfull tool , lets see what devs do with this issue

There is a "mute" button, I haven't tried it yet, but it sounds like it could accomplish what you're looking for in a blocklist.

It's a nice short-term solution, but I don't really think that is a viable long-term solution to the problem. I think at the very least, there needs to be a captcha at login.