This can be done using Thunderbird (
Mozilla Thunderbird) e-mail client.
1) Setup e-mail filters to move whichever items you want to Trash (Tools >> Message Filters)
2) Then right-Click on Trash Folder >> Properties >> "Retention Policy" tab >> Uncheck use my account settings >> Here you will be able to auto-delete anything over 1 message and other settings for mass deletion.
Additionally, you will be using IMAP SSL with GMail via Thunderbird which is debatably more secure, since you won't have to deal with Google Cookies and
webbugs that are the result of their insidious online webmail interface.
If you want to go even further, I would recommend using
GPG with Engimail thunderbird addon so that google cannot snoop out on your e-mail. The problem is your contacts would need to learn how to use it too and it can be confusing at first.