Legal Question in Intellectual Property in Pennsylvania

Simple. Write the words "return to sender - address unknown - no forwarding address" on the envelope and drop it into any US mailbox. The letter will be returned to the sender and it will be up to them to find her.

Your ex-girlfirend is being stupid. Sorry, but there is no other word for it. Lots of things could be showing up in junk mail and if the recipient was a less than honest person, they could engage in identity fraud if these happen to be credit card applications or pre-approvals. However, there is no fixing stupidity.

If you happen to know the postman, you can tell him/her that your girlfriend has moved and not to deliver her mail to your home, but this will not always be reliable. I still get mail after 10 years for the prior occupant of my home and they did forward their mail. So just return the mail with the above notation and be done with it.