You can prevent entire domains' content from being visible to you by blocking domains
Check out the post by @Jonamerica here
For instance, if you didn't want to see anything from lemmy.ml you could go to https://kbin.social/d/lemmy.ml and click the block icon on the right side. (not an endorsement to block lemmy.ml)
You can manage your blocked domains from https://kbin.social/settings/blocked/domains
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This isn't quite blocking the entire instance, but it blocks any posts that are hosted on the instance. It would not, for example, block a link post to youtube that is posted to lemmy.ml