Starting with rev 15 you are able to control the type of account that will be created when the user subscribes to one of your available memberships. This setting can be modified by going to Components > RSMembership! > Memberships > selecting your desired membership > Stock & activation, setting the Enable Changing User Types option to Yes and selecting the user type you want from the Subscriber's User Type option.
You can also change the user's access level when his account expires from the Inactive User Type option or disable the user completely by setting the Disable Account When Membership Expires ? option to Yes.
Please be carefull when enabling this option. If you use your super administrator account in order to test your subscriptions plans, your account's access level will be modified as well.