Turn off iMessage on only one device AND delete all the messages ONLY on that device?

iMessage is installed on three devices, my iPhone, my iPad and my Mac. Truthfully, I never use iMessage on my iPad and, it would be nice to free up that one gigabit of space on the iPad. Yes, it was easy to turn off the iMessage capability in Settings. However, that one gigabit of iMessages is still on my iPad. How do I remove all those messages from ONLY my iPad? The logical answer would be to delete all of them, one by one. Or, use the edit button to check many of them and delete at once. Unfortunately, that is not possible because when you open iMessage, a window pops up requiring you to login with your Apple ID. As far as I can tell, there is no way to close that pop up window. Therefore, there is no way to access all your messages, in order to Delete them. I don't see anyone else posting this question so am I being a bonehead and am I missing something obvious? Thanks