How can I delete messages permanently?

I have an iPhone 4S and I believe the messages and iMessages on my phone are taking up a lot of memory and space! However, I have this problem where when I delete them the app crashes (probably because there is a lot) and I have tried leaving my phone plugged in and leaving them to delete over night, I have tried deleting them one by one in the messages themselves but nothing will work. When I delete them and the app crashes and I have to turn off and on my phone in order for messages to come through and as I do, all of my other messages come back! Is there any other way I can delete them?