CAPTCHA, as that technique is called, is very, very, very, very, very bad for accessibility. It takes only a slight visual impairment for it to completely stump a real person.

There are other ways to do the same thing - one way is to ask a question in a way that requires human intelligence to answer, like "how do you spell apple" or "what colour is an orange". You can change the question every few weeks or months as required.