The way I've deleted an entire folder in the past was to use gmail on my PC. Log in, go to the folder you want the messages deleted from, check the box at the top of the other boxes. That will check all the boxes on that page. Then look at the line above your first email and you'll see a message that lets you select ALL conversations in that folder. Check and hit delete button.
As to the text messages begin full...have you tried rebooting the phone?