This actually takes quite a bit of finesse. The problem with ‘watching’ is that it generally persists per user per topic and it isn’t that easy to downgrade. It is possible but quite tricky via the Rails Console.
I’m not sure how things have evolved in this space, but previously if a tag (for example) results in a topic being made Watching the removal of that tag doesn’t downgrade the watching status. I can’t test this at the mo as tag watching seems to be broken for me.
If however you manage this with Categories, you can control it nicely. Make a subcategory of your desired category, restrict the posting permissions to just admins, and all users to watch that subcategory. To downgrade the watching status of all, simply move the topic from the subcategory to the category.
This is utilised in the #feature:announcements category to some degree here in Meta.