Unfortunately even captcha prompts aren't the way forward..
They will get bypassed through services like deathbycaptcha/2captcha and just make it hard on the genuine users.
If bot owners obtained no profit from spamming comments then they would stop (or really improve their system to the point it looks legit)
A mass flagging operation/blacklist would work but it could end up harming an honest user..
Honestly I think there needs to be a way for a communal suspension (under a certain reputation threshold of sorts)