Having to solve a captcha might help the bot spam but idrk I have no idea how they work.
Will say some spammer groups store the forum security questions in a database too. Sometimes these lists are bundled into their automated software that gets sold and resold again as whitelabel. I think changing the security questions up might help too if a new captcha won't do it.
I think they are just selling it as ad space to fund the website idk