What are my obligations for forwarding mail?

I’ve been at my current address for almost ten years. I’m still getting mail for a previous resident, someone I never met and for whom I have no new address. Most of it’s obviously junk, credit card offers and stuff, but some of it looks fairly official, like medical bills and collections notices and such. Obviously I can’t open it to see what it is, but I’ve been writing “return to sender, no longer at this address” on the envelopes and mailing them back out the whole time, with no apparent effect.

At what point can I give up and start throwing away even the official-looking mail? Do I need to do a “return to sender, final warning” or something before I do?

I just called the USPS hotline (I’m bored, can you tell?) She initially said "Just keep marking it “No longer at this address”. Then she said, if it’s been 10 years, “You can just throw it away, it’s up to you.” (Obviously, I presented it as if it were me that was having this issue.)

So, that’s from the USPS, for what it might be worth.