Provide an accessible alternative if you must use a CAPTCHA
In several recent online and offline discussions on comment spam and other automated, improper use of forms, I have seen or heard people suggest using image-based CAPTCHAs (you know, those images of distorted letters and numbers) to prevent spambots and other programs from successfully submitting forms.
Requiring the user to interpret an image of distorted characters and then enter those characters into a text input field may seem like a nice idea at first. But while it does offer some protection against spam, unfortunately it is also really bad for accessibility.
The nature of graphical CAPTCHAs makes it difficult or impossible for people who are visually impaired, blind, or dyslexic to post comments (or place their order, or whatever the form is used for). Heck, they make it hard enough for people with perfect eyesight to submit the form. If you can't see or interpret the image, you're completely out of luck though.
American Foundation for the Blind have made a nice demonstration of the problems available in a video titled CAPTCHAs on Social Networking Sites Shut Out Blind Users. If you currently use graphical CAPTCHAs on your site I suggest you watch the video and find out what it's like for a screen reader user to interact with them.
Using alternative text, which is the normal method for making images accessible to people who can't see them, for CAPTCHA images is basically the same as removing any protection against spammers, so that obviously does not work.
If you simply have to make your users prove they are human, don't use a graphical CAPTCHA with no alternative. A couple of possible alternatives are logical puzzles and audio, both of which are explained, along with other possible solutions, in the W3C Note Inaccessibility of CAPTCHA. Audio should obviously not be used as the only option either.
Something as simple as adding a checkbox that the user needs to check or uncheck also seems to work well.
Be innovative when looking for solutions to comment spam. But please, be careful with CAPTCHAs, and do not use a visual or audio CAPTCHA only. Provide alternatives.