In my experience, there's nothing you can do besides what you are doing.

The people who used to own my house refused to change their address. Filling out a change of address form for someone else is apparently a felony (tampering with someone else's mail) and I decided not to do it. I forwarded their mail to their new house for awhile and threw a temper tantrum last Christmas, sent them a letter requesting that they change their address, and started returning everything to sender. EVERYTHING. Holiday cards from Israel, business stuff from her freelance clients, you name it.

My address is also linked to someone's license plate who doesn't live here, and I keep getting toll bills. The DMV won't help at all.

Good luck. That's a pain.