Badbot

Badbot is a spam-prevention tool (compatible with all forms on the site). It works by requiring the client to have JavaScript support, throwing an error when JS support is not present. Spam bots cannot handle JS, so this goes a long way towards blocking them without displaying pesky (and unreliable) captchas.

There is a detailed explanation available of this module's specifics.

How to use:

1. Enable module.
2. Navigate to yoursite.com/admin/config/system/badbot and configure the form IDs of forms you wish to protect.
3. Ensure you have a secret value in the Salt field.
4. Set up a contact form or another way for users to contact you if they have JavaScript disabled and get rejected during the registration.