This plugin allows WordPress to block over 95% of spam comments using a time-based hash.
This allows for a minimum sanity check and yet should remove almost all spam comments without the need to sign up to any third party APIs.
The plugin has been tested in real world conditions for around 12 months. It will not block real comments and it is pretty effective against spam, however it only uses a very simple algorithm and for that reason some spam comments may still make it through.