How to reject wrongly delivered postal mail

I have recently had a couple serious-looking letters from the Finanzamt delivered for my ex roommate. What do I need to write on these in order to redirect them to her new address and put a stop to them coming here? I admit I have been lax in removing her name from my mailbox, but yesterday the Finanzamt guy paid a personal visit looking for her which got a little ugly (perhaps he thought I was covering for her, or that I *was* her, pretending not to be!), so I've taken care of that now.

We had a pretty bad falling out and have no contact now, so hand delivery or calling her up is not an option.

Send the letters back to the Finanzamt with following message on the cover: unbekannt verzogen

Well, if she knows where she is, she should tell them. Maybe?

Sorry didn´t read the first mail properly, "no contact" - thought you didn´t know the address, send the letter on and inform the Finanzamt of the new address.

It's not your responsibility to tell the Finanzamt where your ex roomie is. All they need to know is that that person no longer lives with you. The "return to sender" approach may/may not work. Ultimately, they can't do anything to you since it's not you they are looking for. Take the name off the mailbox. The rest has nothing to do with you.

Hmm, ok. I hate to waste a stamp on the zicke but I guess it must be done.

You don't need to use a stamp, just cross the old address out and write the new one on, then place in postbox. Easy.

I had the situation last year where I was getting serious-looking mail delivered simply because the last name was the same as mine. Being an English name it's not so common in this part of the world, but the first names were different. I suspect they'd just looked up the name in the phone book and decided to threaten whoever happened to live at my address in the hope that it might be the right person.

Anyhow I sent the first letter back to the sender (no stamp necessary), simply marked as "unbekannt". The second I wrote a note on the back explaining that it wasn't me and they should please get their act together. The third one I took to the post office and got them to return it (still at no charge) which they did. I have heard nothing since.

Had a friend get a letter from the Fianzamp as well complaining about how his taxes weren't paid blah blah blah and he couldn't figure out why untill he realized they had the right name but wrong address turns out there were two of him in Frankfurt.

cross out the address, write "unbekannt verzogen" on the envelope and just drop it into a mailbox. no stamps required.
if you want to make sure you don't keep getting mail for her, you might also put a sticker with her name and - again - "unbekannt verzogen" on your own mailbox for the mail delivery to see.