D'Arcy Norman

A very simple mu-plugin to automatically (automattically?) insert a “Spam prevention powered by Akismet” notice (with hyperlink to Akismet.com) in the footer of each blog hosted on a WordPress Multiuser service to abide by the licensing terms of Akismet. Adds a small “Spam prevention powered by Akismet” to the footer of every blog to abide by Akismet licensing terms.

Akismet Credit Inserter

Adds a small "Spam prevention powered by Akismet" to the footer of every blog to abide by Akismet licensing terms.

Per Søderlind

Based on a users IP address, WP-DenyHost will block a spammer if he already has been tagged as a spammer. Use it together with the Akismet plugin. Akismet tags the spammer, and WP-DenyHost prevents him from accessing you site.
If you have a <a href="https://www.cloudflare.com" rel="nofollow ugc">CloudFlare</a> account, the plugin can add spamers to <a href="https://www.cloudflare.com/threat-control" rel="nofollow ugc">CloudFlare Block list</a>

WP-DenyHost

WP-DenyHost denies a spammer from accessing your WordPress site. WP-DenyHost can add spammers to the CloudFlare Block list

Akismet Credit Inserter vs WP-DenyHost