[tialaramex]

Cross through the wrong address. Every year or two somebody from the management agency tidies the noticeboard for the building I live in and removes my hand written sign explaining how this works. Then, next September/October when lots of people move in (some fraction of the occupants are students) the noticeboard gets envelopes pinned to it with undeliverable mail. I write a fresh sign.

The sign is a flowchart, it says first, is this mail for a different address? If so, either redeliver it (duh) or write "Misdelivered" in bold leters and put it into any postbox.

If not, but you don't recognise the recipient, strike through the whole address in black pen and write clearly "Not at this address" then put it into the postbox.

This won't stop you getting more mail by the way, I still get letters labelled "Urgent" with the name of the previous owner years after I bought this place. But it does stop literally the same mail coming back since the OCR will reject the crossed out address -- it's just that the sender may not have any effective process for what to do when they get the mail back undeliverable.