Hi --- On a few sites I have, I do have humans signing up hoping to spam! So with or without captcha, it's a problem... They can select as many "affiliations" as they want (zero or more), and since last week they always select two. I don't think that small site has anybody writing a bot for it
What I would do as a work-around: Make a human-readable question in the right style for your website (instead of captcha, only for the free plans), and disable those that give the wrong answer. Make it optional but write IN LARGE LETTERS (CSS override?) that it isn't optional actually --- humans will read this text, bots will skip as optional.
Style of question is serious or funny --- different for a maths students website, a law office, or a car enthousiast website. For example, for car enthousiasts display photo of the back of the car and ask "Is this car a: A/Lexus, B/Toyota, C/Harley, D/Ford, E/Jaguar"... [Or ask about the car in the logo of the website, or whatever.] If you get a wrong answer, then move to a Joomla group "Spammer" or so. Leave a text-field next for comments; you can periodically check if you were too harsh.
Do take care the language isn't a problem in the question (not all may speak the same mother tongue as your website)! Don't ask questions about colours, as about 5% has problems with red/green. Etc.... think a bit about the question.
Alternatively, write in text "Humans, make sure you select the one-before-last option!" then ask whatever (impossible) question in a dropdown, only activate accounts that select the penultimate option.
