SpamPot
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Adds a honeypot form field on the registration and login pages to trap spammers.
Blocks spam signups by adding an invisible field to your registration and login forms, and switching its id with the real username or email form field.
When spam bots blindly fill the “required” fake field in, WP will redirect them to an error message and not let them register or login.
No 3rd party service, no intrusive captchas.