For some odd reason, on my Only spawn, it still creates a neutral team even though neutral is off.
If you have at least one Team (a "Cops" team, for instance) inserted in your Teams Service the game will automatically create a Neutral team whenever a player who's not in that team spawns.
In order to prevent it you will either have to delete the "Cops" team (and every other team in case you have any left) or the Team Service itself, or you could make it so your Only spawn belongs to a team which could be named to something neutral-ish (like "Citizens", following the example above).
Hope this proves helpfull!