how to disable comments (and hide those already posted)

You would need to edit your theme’s template files and remove all references to comment template tags.

can anyone else provide more detailed instructions?

the typical template tags or functions related to comments are:

comments_popup_link()
(most likely in index.php and archive.php)
and
comments_template()
(most likely in single.php and page.php)

possibly with parameters in the brackets.

there is really no alternative to editing each of your template files in your theme, and looking for the above functions (and possibly anything with the word ‘comment’ in the code) and to remove these together with their corresponding html tags.

thanks so much!