Actually captcha these days is nearly full proof. Bots can't select street lights or click on buses in the captcha photos. The captcha of 2005 was easy to defeat which was just a simple 4-6 letter captcha but these days it's full proof.
I wish that was true but unfortunately, Google have shot their own reCaptcha to pieces by pioneering picture recognition technology. There are still a lot 3rd world captcha solving farms using humans but the more sophisticated bots can solve them automatically.
Captcha is only an obstacle to small scale amateur abusers.