Many years ago, I had a mate who dealt with letters received by the Complaints Section of Telecom (before it became Telstra).
They received many legitimate letters ("Why do people write to you instead of phoning?" "Because we've stuffed their phone lines"), but also many of tin foil hat levels of craziness.
They used to deal with the latter by attaching a sticker to the letter and, if the sender had included an address on planet earth, posting it back to them.
He gave me a sheet of the stickers, which read
Thank you for your letter. Please do not write again.
He had a variant printed up for offensive and abusive crazies as well, but his boss wouldn't let him use them.
