Accessible portal registering solutions

Good afternoon:

I´m working in a web application which allows user
registering, login and logout. I´m looking for a
secure way for the register process, avoiding massive
automatic registers via software. For example, in
Yahoo you need to read a random word painted in an
image to register mail, but it seems a bad solution
looking from accesibility and WAI guidelines. I wonder
if anyone here had been working about it or anyone has
any idea.

Thank you.

Pablo Enríquez.

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