Anonymous users are keep submitting register form

Hi,

I have the registration form at http://www.xxx.com/user/register.

Anonymous users are keep submitting the form at the above url to create account. I did not have the register link on my site but I am not sure how people are finding the url. Accounts are being inactive. I have to delete all every day. How can I stop them submitting the register form?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/22/feed