Go to your
All mail view,
and click "Select all" (or the drop down option All from the checkbox far left of Archive | Spam | Delete buttons).
Underneath the | Archive | Spam | Delete | + | - | Move to Inbox | Labels | More |
buttons...
You'll see a message appear in a yellow bar that says:
All 100 conversations on this page are selected. Select all 73795 conversations in All Mail
Click on the "Select all conversations" link on the right side of the message, then click delete.
The actual deletion process may take a very, very long time, during which you will be unable to use Gmail or log in (don't worry, it's still churning away in the background, even if it gives you an error!)
When it's done, all conversations (except spam) will be in the trash and you should see a message that says "You don't have any mail! Our servers are feeling unloved."
Now just empty the trash and delete all spam, and your account is now empty.
Best Answer
Check out this page on mail.google.com