Hello,
You will need to set 'CMS Desk -> Tools ->
E-commerce -> Configuration -> Store Settings -> Allow anonymous customers' setting to true first. Then you can modify second step of check-out process as you need (e.g. remove options for other registration types) - you can find its source in ~\CMSModules\Ecommerce\Controls\ShoppingCart\ShoppingCartCheckRegistration.ascx(.cs) files.
Best Regards,
Martin Dobsicek