Yes, you can return unwanted mail to sender. If the mail has your name and address on it, then it is best to write "Refused" on the envelope or package. This lets the post office know that the name and address are correct, but you simply do not wish to receive the mail. You can take this route with junk mail or accountable mail such as certified or priority mail. If you have already signed for the package or opened it, then it is too late to refuse it at that point. You will need to repackage the item and attach new postage to get it returned.