How do I avoid unwanted users create accounts? I am using Drupal 7 and use the User verification module for blocking (prohibiting) unwanted users in our site, but a user is creating an account every second.

How can I stop that from happening?

Is it possible to first send an email to the users, with a verification link  included? When that link is clicked, users will be registered on the site.
It depends on what kind of control over user accounts you want.  By default Drupal allows visitors to register new accounts.  If you want to disable that, you can set Who can register accounts? to Administrators only at /admin/config/people/accounts.