This is a real problem, that client devs should address, by starting every new user off with a curated, publicly-readable relay feed that allows anyone logged-in to the client to zap and send reactions, but not necessarily note, reply, quote or DM. You can also include a publicly-writeable relay for creating profiles and publishing notes, but don't necessarily make that the first feed newbies see, when they open up the app. They should have to purposefully opt-in to smut, junk, and spam. And they should have to pay or apply for access to the default feed. Have, in other words, a client inbox/outbox model. They can use that, or define their own on their profile, and then switch to that.