Asklemmy

You have three tools in your instance settings at https://lemmy.world/settings: the Show/Blur NSFW Content options in , and blocking users/communities/instances, setting your feed to only view Subscribed communities and just don't subscribe to any NSFW ones. The first is dependent on posters ticking the box to mark their post as NSFW. If you see a NSFW post that is not marked as such, report it. Admins on their instance should address the matter. The second option isn't NSFW-focused in any way but you're in control of it. You could block instances that are primarily intended for NSFW content, such as lemmynsfw.com, and then block any individual communities on other instances, and lastly if there are specific users who just seem to refuse to stop posting unmarked NSFW content and their instance admins won't do anything about them you block those users.

You probably can't expect to fully pre-emptively block all NSFW content, because someone might post something without marking it correctly. Could be by accident, could be they just don't care. But reporting them gives a chance to fix it rather than blocking an otherwise safe community or user.