I think if you’re looking to hide just one topic then you can probably do that with CSS. My CSS is poor, but I can see the data-topic-id in there, so it can likely be targeted?
Ah, there’s a gotcha! in there. If you repurpose it then that one doesn’t show up in some topic lists or fire notifications when people reply, etc. I think the best thing to do with them, as is, is just to leave them as category descriptions and close them off (and unlist them if you didn’t want them visible to most people). Dev tweaks aside, obviously, if you can swing it. 
Storing the category description in topics has cropped up as an internal discussion, as well as the way we handle ToS and Guidelines, etc. No immediate plans to change the format though, but I think there’s definitely a conversation in there to be had.