How to control new user registration in WordPress:: https://www.todhost.com/knowledgebase/838/How-to-control-new-user-registration-in-WordPress.html

One problem plaguing WordPress especially membership sites that take user registrations is spam user registration, which can also be referred to as user registration spam or spam bot registration. This occurs when user accounts that are spammy are being created at regular interval. If your WordPress site allow users to register, then it is like going to be vulnerable to spammers.

Read full article: How to control new user registration in WordPress:: https://www.todhost.com/knowledgebase/838/How-to-control-new-user-registration-in-WordPress.html