You can charge them with a crime, but leaving specifically objectionable content up is rather pointless because you're not litigating whether it's specifically prohibited (as calls for genocide are in Germany) but whether liability attaches to the apparent speaker/writer (whose account might have been compromised, for example).

Another reason not to leave it up is because it's frightening to the subjects of such eliminationist discourse, who shouldn't have to put up with being considered acceptable targets for systematic violence.