Drupal sites are subject to web-bot registrations that target Drupal's /user/register url.
The Members Page module was crafted to fight this nuisance by creating a dedicated members page and registration page. The latter is available even if /user/register access is blocked. For added security, both page locations (url's) can be changed. Web-bots simply don't know where to look.
Full details here: http://sontag.ca/drupal/members-page-module
Available for versions 6, 7 & 8.
You should be able to test this module out in less time than it takes to read the web page that details it. It requires no other modules and completely removes itself on uninstall.
Any feedback would be greatly appreciated. Currently in the queue awaiting approval and testing by the community.