How can I deny access to a certain site URL?

You can block a certain URL on your site through the .htaccess file

This file must be in the root of your site, if not create a new file.

SetEnvIf Request_URI "/administrator" block_admin_area
Allow from 192.168.0.1
Deny from env=block_admin_area

This blocks access to /administrator for all visitors except for IP 192.168.0.1 – which you can replace with your own IP address.