For the topic Announce there really should be no replies. If a question or such is needed, then a new topic should be created with a link and/or tag to the announced item.
If applied the current replies would be moved into new topics so as not to lose them.
The conversion of the categories Announce and Useful Code is done. All topics now have 0 replies. For any topic with a reply the replies were moved to a new topic. While Discourse will not allow me to configure it so that there is no reply button, any reply in the Announce or Useful Code category should now be trapped by the system and placed in the moderator queue for action.