Currently it's very confusing for new users to turn off comments for pages they created already.
You can disable comments for all page in settings but in doesn't work on pages already created.
For particular page you can turn comments of via Quick Edit link but this is option is hard to find for new users - only shows when hovering over page title.
On the Edit page, "discussion" block is hidden by default and the only way to show it is to change "Screen Options" - which is also hard to find for new users.
I understand the idea was not to overwhelm users with all the options but it's counter-intuitive that Quick Edit has actually more options (by default) than full Edit page.