Team billing issue

I'm trying out Spark and followed the instructions to allow team-billing only. I've set up my plan below.

        Spark::teamPlan('Basic', 'provider-id-1')
            ->maxTeams(1)
            ->price(20)
            ->features([
                'First', 'Second', 'Third'
            ]);

Somehow every user can still create multiple teams without making sure there is a valid subscription up front. Did I configure something wrong or am I misunderstanding the way it should work?