Hello
Yes, yellowballooons is right: you have to opt in to see the SN topic areas. (This is because people were clicking on threads in Active Conversations without realising that the threads were in the SN topic areas, and were offering advice that was unsuitable for children with SN, leading to many a bunfight - and requests from SN posters that the topics be made opt-in.)
So we suspect that the 'hide' option in Customise was left off because you had to actively opt in to see the topics in the first place, so it was thought that once people had done this they wouldn't want to opt back out again (maybe - this was all a long time ago...)
We'll see whether this (the 'Hide' option on SN topics in Customise) is something Tech can fix for us.
You should, however, already be able to hide individual threads within SN?
An SN Active Threads box is a good idea but a more complicated Tech fix, we suspect; we'll have a think about it, though.