Humph. I just tried following Rusty's instructions since they are obviously vastly superior to mine but there is no link to manage hidden threads at the bottom of my active conversations. I suspect a plot.
I couldn't remember if I was meant to have any hidden threads on the list, so I just hid the chocolate tombola thread as an experiment. It has indeed been wiped off the face of the earth. Now I'll never know where to buy cheap chocolate