Thanks, this pointed me in the right direction.
The (Lithium theme) Disqus template starts with:
{{ if and (not (eq .Site.DisqusShortname "")) (not .Params.disable_comments) }}
As I now understand, the .Params.disable_comments is a page variable that can be set in the page's front matter. So adding disable_comments: true turns off comments for that page only.