Hiya!
There is a hint on the page:
This category is public, everyone can see, reply and create posts. To restrict permissions, remove one or more of the permissions granted to the “everyone” group.
So to remove the “create” permission, you can remove everyone, and then add everyone with the settings you want; I tested this just now on 2.9.0.beta3 and it works…
In that screenshot (
) the “New Topic” button is non-functional.
However… when I refresh the permissions on a category, it shows the same “This category is public…” message, and all boxes are checked, while the intended permissions remain; in this case all three checkboxes are checked, but the category in question only allows reading and replying, but not creating new topics.
I’m going to rebuild my site and test some more, to see if this still persists. 
Two words for you my friend: safe mode. 
