This plugin provides you an easy captcha without javascript, without images, without math. Only a few of CSS and a few of server side. 
An hidden text input is placed in comment form a human cannot see it so a human cannot compile it, a spambot otherwise see the text input and is forced to compile it. 
The plugin check the existence of this text input and check if it has a value or not. It drops all comment with a value in the text input as spam. 
Really simple and powerfull antispam system. Low CPU usage and no access to DB.

Hiddy captcha 1.5.1

This plugin provides you an easy captcha without javascript, without images, without math. Only a few of CSS and a few of server side.

Payadl

BT Captcha – simple, Bilingual, Flexible, Protect Your WP Blog Against Comment Spams 
Special thanks to the developer of <a href="https://wordpress.org/extend/plugins/anti-captcha/" rel="ugc">Anti Captcha</a> Plugin

BT Captcha

BT Captcha - simple, Bilingual, Flexible, Protect Your WP Blog Against Comment Spams

Hiddy captcha 1.5.1 vs BT Captcha