How can I delete ALL email at once, rather than one at a time or one page at a time?

Depends on what company you have. On Comcast, you click on the left box on the header bar. Makes all blue color, click the trashcan icon.

My crystal ball isn't telling me what email application you are using or what email service you are using. Because there isn't a one size fits all answers... in fact in many (at least the ones I use) you can't do that at all.