I've never tried to do an opt-out for someone else. A few places like dmachoice.org have web forms but I made a lot of phone calls.

There are a couple of local advertisers that are going to send their stuff no matter what I think. I do still get a small amount of junk.

You could try crossing through the address, writing 'no longer at this address, return to sender' on it and dropping it in the out-going mail.