Actually, that’s two separate things. There’s a firstjoinkit, which I believe can be automatically delivered to new players. Stopping them from modifying their inventory is a lot harder - even Adventure mode is friendlier. That would rule out pickups, drops, crafting, chest interactions, and maybe even eating food. I’m not sure I know of any plugin with that kind of extreme inventory lockdown.
I know there’s a way to force players to return to spawn on login (nucleus.spawn.other.offline) but I’m not sure if there is a way to force it globally. I’ll have a rummage in the config file…
If true, players will be sent to the default world spawn on login, unless they are sent to the first login spawn, or they have the “nucleus.spawn.exempt.onjoin” permission.