arijose

This plugin was created to stop human spammers. Captcha or other image methods do not work when an actual person is spamming your site. This plugin helps to resolve this issue. It allows your to create a custom question and answer on the registration page of a wordpress site that only the users in your demographic can answer. An example (and the reason I created this plugin) is a school track website. I would get up to 3 to 5 bogus registrations a day. I added this plugin with the question “What city is your school in?” and that number went to 0. Someone in another area is not going to know this question, however anyone who would/should be registering would defintely know the answer. This plugin will not work for everyone but it certainly was a perfect fit for me and I know it will help others too.

ARS Reg Secure

This plugin helps block bogus registrations by allowing a custom registration field and answer.

willjenkins

Using the most effective features of other spam programs. Astounding Spam Prevention effectively guards against comment spam and registration spam without the use of a Captcha. 
This is a fork of the original Stop Spammers Spam Prevention and uses some of the code, but it is greatly simplified so it is not as aggressive and will not prevent a user from logging in. It does not interfere with jetpack and does not know about WooCommerce so there are no conflicts. Unlike Stop Spammers it does not check logins, but only checks registrations and comments. 
It also repairs many bugs that I found in the Stop Spammers plugin and has many new methods for detecting spam.
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See my books and contacts: https://linktr.ee/keithpgraham

Astounding Spam Prevention

Based on Stop Spammers, this lightweight spam stopper is still aggressive enough to stop 99% of the spammers and robots hitting your site. It is careful to never lock a webmaster out of WordPress. The options interface is slick and the logging is very very useful. 
I use it on 11 of my sites and I have my clients all using it.
Thanks to Will and all the effort he put in to make this plugin extremely useful while still being very very safe.

Works like a charm

It kills the spam

Got dozens of registrations after installing and activating all options. It made no difference what options were set. Uninstalled.

Made no difference to the spam

Very effective anti-spam plugin that eliminates comment spam, and registration spam. Combines many effective methods for identifying spammers and keep …

ARS Reg Secure vs Astounding Spam Prevention