Captcha

I literally just did this!

Add this: https://wordpress.org/plugins/si-captcha-for-wordpress/

You can have it display only on registration, or on login too. Works like a charm.

Pic: http://www.dropshots.com/sydneymc/date/2015-02-25/11:51:56

Install GoodBye Captcha – works like a charm for me and… no need for captchas.
https://wordpress.org/plugins/goodbye-captcha/