It is very true that after deleting the text messages from iPhone, the text messages does not get deleted permanently. One can easily recover those messages from your iPhone with the help of suitable iOS Data Recovery program.
However if you want to delete your messages permanently from your iPhone 6 then you should use iOS Data Eraser, it will help you to easily delete all messages from iPhone. After using this software, all the text messages will be deleted permanently.
Here Find out step by step guide on how to delete text messages permanently from iPhone.
It will delete all data from your device such as your photos, videos, contacts, browsing history, audio, call history and more.