My website can only be accessed by authenticated users. How can I automatically make the author's name the user's name without giving users the option to input anything else in the name field?
If you go to the permissions table, located at admin/user/permissions you will find a permission that allows you to disallow users from changing their own name it is called change own username
If you are manually creating users, this should work well for you because they will have the name you created for them and they will not be able to change it. However, if the users create their own accounts, one option for you to consider is the Real Name Module