If you don't want to allow users to register from your site frontend, then you can disable registration for your site.
In order to disable registration you need to edit each user group in your system from Users / User Groups section and uncheck the option Auto Register. This option allows a guest to register as a user belonging to that group by filling in a registration form.
If no user group is allowed to register from the site frontend, the Register link will disappear. You will still be able to add users from administrator panel Users section.