Permanently deleting Email from Default Email app
From the Android stock email app, you can permanently delete emails if you access the trash folder and delete them from there:


Select and email to delete and touch the delete icon;
Access the Trash folder;
Repeat step 1 inside the trash folder for the same email.


With this steps, the email gets permanently deleted from your account.

I've tested this steps just now to be sure. Anyways, I did not find any setting within the APP or account preferences, in order to do this more seamless!