From what I understand about Lemmy, the only way to disable signups completely is by defederating. Meaning, you would not be able to view other communities from other Lemmy instances like .world or beehaw.
To get around this, you can switch to application signup like beehaw and just ignore/reject every application but this would also mean that anyone can view your instance as a guest. Like you can go to lemmy.world and view posts and comments without logging in
Atleast, that’s what I understand. Maybe someone can correct me if I’m wrong.