I added reCAPTCHA to account registration, comments sections, etc., made a little announcement banner above the navigation menu, and deleted 12,600 member accounts. I never made the registration link for the website public prior to today, so all 12,600 accounts were probably advertising bots that simply advertise their own websites and products by making comments and threads.
It looks like some advertising bots are able to get past reCAPTCHA. The website currently receives about 600 comments per day from them. Bots that make accounts seem to not post any comments.
I might require website visitors to create accounts for making threads and posting comments in the future, but before I do this, I should make “Register” and “Login” buttons that are clearly visible on every page. Also, if I decide that I want to have more social interaction between users on the website, I might purge the members list again and make a /members/ or /community/ webpage that lists users registered on the website.