For a bit more explanation of the reasoning behind this question - a forum I am a part of has recently been dealing with a transition to Discourse, and in doing to the visibility of forum games has shot way up due to the latest post being the ones shown by default in pretty much every conventional way of viewing the forum.
While these games have been around for a long time, and administration does not wish to outright remove them, the old software simply didn’t show these categories in its latest feed by default. Thus, we wish to accomplish something similar here.
In investigating the statistics of the site, it was found that roughly half of all traffic comes from unregistered guest accounts. Therefore, simply muting these categories by default for new users has not resulted in any significant improvement of the situation.
Thus, we are hoping that it is possible to mute these categories for unregistered users somehow, and then we can simply pin a post telling new users with accounts how they may mute these categories on their account if they wish.
Have you looked at category permissions? If you go to a category page, click “edit” and then the “security” tab you can set permissions for what user groups can see, reply, and/or create topics in that category.
In this case, sounds like removing the default permissions setting (everyone) and adding a rule so trust_level_0 users can create / reply / see the category might do the trick. You could also e.g. make it so tl0 (all users) can see, but only tl1 users can create / reply, or something like that.