Stop Spammers plugin
There are malicious spam bots out there searching for WordPress sites open for registration. These bots can add spam accounts into your WordPress site which could be a threat to your site’s security and credibility. To prevent from spam registrations in WordPress, install Stop Spammers plugin.
After installation and activation,
go to stop Spammer in the left menu;
Select Diagnostic
Test option
You can keep all options as they are.
Link