How to delete iOS messages so they can't be read online?

First of all, I have an iPhone 5s with sprint and my parents are somehow reading all my texts even if I delete them on my phone. Can anyone please tell me how to delete them permanently, I really can't say anything over text without getting in trouble now. I know people say the phone provider does not show texts online, but my parents either have an app or are reading them online.

Sprint does actually provide a service for capturing messages as an additional purchase with the ability to view them online. Not sure if it's 100% compatible with iOS, but the option is there.