This did pop up here as well:
At the moment we don’t have any way of “dismissing” new when you are drilled in to a category. I think this can be important if you are working through a bunch of categories and simply want to “opt out” of a few of them.
Technically the only thing we can do is set 1 as last_post_seen on topic_user for all the new topics in the category. It means these topics will not be tracked and you will simply have “opted out”.
User at gearbox was also asking for a 1 click dismiss of both new and unread.