I'm pretty sure insults fall under free speech protection in the US, and in fact this case/law has been overruled by higher courts for this reason.
As long as you're not committing slander or libel, you can insult anyone for anything really. And slander/libel is very hard to prosecute as you have to prove that the claims aren't true and they they also caused tangible damage.
Blocking Lemmy.ml, the oldest and one of the most active, seems more than extreme.