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There’s the presence enabled admin setting if you’d like to turn it off completely?

After a little exploration, I believe you can use a touch of CSS to target hiding it for a specific category. I’ve only given it a brief test, but it seems to work as expected (replace SLUG for the slug of your category):

body.category-SLUG div.presence-users {
    display: none;

As it’s CSS, it can be ‘unhidden’ using safe mode or the browser inspector, but may be enough for your use-case?