I don’t on Lemmy because it doesn’t really do anything besides hiding their posts. On Mastodon I block people who are AI trolls, political trolls, reactionaries, and in general people who are hostile. People who are objectively horrible people, sometimes it’s because I see them doing it, but other times because they lashed out directly at me. Most blocks on Mastodon are preceded by reports to server admins but I still block them because it usually takes time for people to be moderated.