How to disable State/Province in billing address blocks
We also needed to disable some billing fields, including the state/province, so we wrote an extension for it: https://github.com/WeMoveEU/nobillingfields

It keeps only first name, last name and zip code, and this is a hardcoded thing, but you should be able to easily adapt to your needs if you have a developer. 
Note however that when we migrated to 4.7, the extension stopped working, and I patched civi core to make it work again. I reported the problem, and the suggested fix was to unset validation errors with a hook, but I have yet to try it.

I'm afraid there is no easier solution...