It doesn't even pull them down on your phone unless you go to the Trash label, so they're not on it.
On your Google Account, It doesn't keep them forever, it automatically deletes them.
But if you absolutely must go in and delete it forever on the phone (and it can't wait until periodic visits to your computer), use the browser to go to mail.google.com. The Mail for browser is different from the Gmail app, and it has every menu choice avialable on a web browser.