Marketing mail won't get returned if you do "Return to sender." It is sent bulk rate, which doesn't provide a return to sender.
I'd enter that person at catalogchoice.org (assuming it's magazines).
Regarding DCPS, can you contact the specific school? Say "Larla family isn't at this address. Who do I call to request that Larla Family be crossed off the mailing list?"
Also, I wouldn't worry about "getting someone in trouble." This is a lesson in Being An Adult 101.