This extension allows you to select which regions are available to be used for customer address creation.
Forked from Tomas Novoselic's extension, updated to work with Magento v1.9.1.
It works only for US.
Keep in mind that this extension is created for purposes of blog article at inchoo.net and as such can only be used as proof of concept. If you find it helpful in order to learn Magento, that is great, but be careful if you plan to use it in production environment.