Here is little extension called Inchoo_USDisableRegions.
If you have any reason to disable some specific regions from US country, but you don’t want to delete them, this extension will allow you to chose which ones are possible to use for your customer’s addresses.
You can get it here: https://github.com/tomasinchoo/Inchoo_USDisableRegions/tags
Extension is tested only on Magento ver. 18.104.22.168
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.
So, that’s it, just install extension and go to System->Configuration->General->Enabled Regions and chose ones you would like to use.
Please note that this works only for United States regions.
Let me know if you have any additional questions 😉