Spammers apply for an account, how to disable other users from applying for an account on my website
There are several options. In addition to disallowing vistor registration (Navigate to Administration → Configuration → People → Account settings and select "Administrators only" under "Who can register accounts?", you may you one of several contributed modules that is designed to prevent spammer and spambots from registration.

My personal favourite is Mother May I.  It adds a field to the registration form where the registrant must type in a "secret word". If the registrant gets this wrong, no new account is created.  This is perfect for invitation only sites, where the invitation reveals the "secret word".  However, I find it works pretty well on other sites as well, where the "secret word" may be the answer to some a "riddle" that is solvable by humans but not by 'bots.

As an alternative to the above, there are are several contributed modules that let you add various types of CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) challenges to forms, including the user registration form. These block spam robots, but do not block human spam registrations. However, blocking 'bots take care of about 99 % of spam registratons, so doing this helps a lot.  The most popular module providing a CAPTCHA is named CAPTCHA. It comes with a built-in challenge, but is really a framework that to manage a set of CAPTCHA plugin modules, letting you experiment with these to find one CAPTCHA method that is effective for your site.

There are also contributed modules that provides a Drupal  interface to some external third party anti-spam service. The most effective of these is http:BL. an interface to the project Honeypot blacklist.  I've found that this module provides efficient blacklist lookups and blocks malicious visitors effectively.  (There a few false positives tho', in particular if your site attracts visitors that connects through TOR.)