I don’t think this is possible through DNS filtering because it is your home lemmy instance showing you the results. You could block specific communities, turn off NSFW posts in your profile, or use a filter in Ublock Origin. These are all client-side solutions and would not auto-apply to a whole network. I don’t think there is a feature to block posts from a whole instance without being in charge of your own instance and de-federating. I would wager a per-user instance block will be coming down the pipe eventually.