Hi,
If you set the "access level" of the shipping method to "public", then it won't be available after the customer validate the registration form.
So this should not happen.... unless you've changed the checkout configuration to use the "guest" mode.
In that case, even after the form, the customer will still be in the "public" group.
In that case, you can use plan B:
instead of restricting the shipping method to a user group, you can instea restrict it to a zone like for example a fake state zone that you can create in the menu System>Zones.
Then, select that zone in the "main tax zone" setting so that this zone is selected by default before the user enters his address.