You can't and you shouldn't have to. As long a people keep responding, that "signals" some interest. I find a lot of the topics are ones I have no interest in...but wouldn't want a mod to have to select for s/n ratio. That's in the eye of the beholder. As in most forums, lack of clicks on a thread eventually wins out. Then (months or years later) some mope who can't read a date revives a defunct thread.
That's the price you pay to avoid edited and censored sites. Just "filter" with your head and fingers.