drop it back in the postbox with a note like 'get it right next time'. Its their job not yours to deliver 
This is what I do, it happens all the time round here (London). I get post not addresses to us at both home and work. I just chuck it in the nearest red post box, and it gets re-delivered.
How do I know? Because when I moved to the new office, I kept getting mail for the previous tenant who left no forwarding address. So I used to write on it 'moved away, no forwarding address, return to sender', and chuck it in the post box.
The next day I would get the letter again, with my hand-written note... apparently the post office uses some sort of electronic sorting system that hones-in on the post code, and the postman can't read or something.
The only way to get rid of these letters for good (either than by burning them...) was to write the note but also erase the house number with a permanent marker. I never saw these ones again, though I cant say for sure if they were actually returned to the sender.
What bothers me more however, is that some of the post that is sent to me regularly go missing, and I wonder if these letters get to the wrong address, and then chucked in the bin... :-/
Right now I am holding my breath for an item I sent by post to another OOF member, which did not arrive yet.... it is not valuable as such, but it is irreplaceable... maybe it will arrive on Monday.