If you get someone's mail delivered to your address it could be that the person listed on the mail used to have your address and hasn't updated the sender on their new address.
So what you can do is write on the mail return to sender and then put on the mail that the person named on the mail doesn't live at this address anymore.
So write return to sender this person does not live here at this address anymore.
Then just place the mail back in the mailbox and the mail should be sent back to the sender.
Sometimes I get my neighbors mail and I just deliver it to them but I also sometimes get someone else's mail that used to live at this address and I always just write on the envelope "Return To Sender" and that the person listed on the envelope doesn't live here anymore.
The mail always gets sent back to the sender without a problem.