jamiechong

A lot of registration spam comes from emails with a subdomain. Spammers control a domain such as buymyjunk.com and then attempt to register on your site with emails such as irenemwd@foster2.buymyjunk.com and jakemgm@foster7.buymyjunk.com. This plugin simply blocks all registrations using a subdomain.
Reputable users will have a normal email address from a reasonable domain like @gmail.com, @mylegitbusiness.com, @outlook.com, @myschool.edu, etc.
There is no configuration and no setup. It simply just works. The plugin logs all blocked registrations so you can find potential false-positives. Find a link to this log file on the Plugins page where you activate/deactivate this plugin.

Ban Subdomain Emails

Prevent people from registering with emails that contain a subdomain to help reduce spam.

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

Ban Subdomain Emails vs WP-DenyHost