My family owns a condo, and I recently moved in. The previous tenant refuses to have his mail forwarded because, as he says, "I don't want the junk mail, and everything important is forwarded already." I am getting multiple catalogs and letters addressed to him and his wife still. Can I fill out a change of address form for this guy who just wants me to clean up his mess rather than unsubscribe himself?
There is a USPS form to fill out, to change a person's address. This page shows you what the form looks like. You should notice the text which they transcribe onto the page:
NOTE: The person signing this form states that he or she is the person, executor, guardian, authorized officer, or agent of the person for whom mail would be forwarded under this order. Anyone submitting false or inaccurate information on this form is subject to punishment by fine or imprisonment or both under Sections 2, 1001, 1702 and 1708 of Title 18, United States Code.
So you cannot legally do anything, other than take annoying mail and notate it with an indication that they are not at this address ("Return to Sender" is the classical response), and stick it back in the mail.
It is also legal to go to the post office and complain (politely) to the staff, who deal with the situation in a way that doesn't put you at legal risk.
As the answer by user6726 explains, you cannot legally fill out a forwarding order for someone else without that person's permission.
However, you can write "Please forward to: [Tenant's new address]" on the mail if you have the the new address at hand, and return it to the carrier or the post office with this note, if you choose. (Cross out your address with a single line so it is readable.) If you return it to the carrier several times, s/he may filter out mail addressed tom the former tenant. Or the tenant, finding mail forwarded in this way, may choose to unsubscribe from no longer wanted sources of mail.