This article explains how to quickly empty the Trash and Spam folders in Gmail. Additional information covers how to permanently delete an email. Instructions apply to current web browsers and the iOS Gmail app.
How to Automatically Empty Gmail Trash
Here's how to empty your Gmail account's trash:
Select the Trash label. You can find it under More, in the left sidebar of the Gmail screen.
Select 'Empty Trash Now' at the top of the Trash messages.