Install Reddit Enhancement Suite a.k.a "RES" and navigate to "Comments > Hide all Child Comments" and set "Automatic" to "on"
More info here /r/Enhancement
I highly suggest downloading Reddit Enhancement Suite if you haven't already. It gives you a way to customize Reddit exactly how you want it.
Then go to RES settings console > Subreddits > filteReddit (filteReddit) > NSFW Filter (NSFWfilter). You'll find the settings console (the little gear symbol) in the top right-hand corner.