I once used a form with a random math question: 7+2 = __
It prevented 100% of bot spam for years. Granted, I was never a big enough target to make anyone rewrite their bot, but that's the same for most of us. I'd never use a captcha so long as something trivial like that works 100%.
This (or something very similar) is a core feature of phpBB:
https://www.phpbb.com/support/docs/en/3.2/kb/article/how-to-...
It works very well.
If you want to filter comments you could even make the questions reflect the content of the article, filtering uninformed TL;DR type of comments and giving the users the feeling you value onformed opinions.