A friend once complained about how much unsolicited mail he received, and claimed that he'd got his revenge on the Readers Digest, which had resisted his attempts to stop them sending him stuff. He said he'd taped their reply-paid envelope to a breeze-block, taken it to the Post Office, and posted it back to them.
This would have been in the 1990s. We're talking about one of these:
I think he wrote a nice note with it, along the lines of "I requested that you stop sending me unsolicited mail. Please accept this concrete block as a token of appreciation. Love, Joe Bloggs" etc. The implication is that the recipient would have to pay a huge fee.
I was recounting this tale, and as I spoke, my BS detector alarm went off. The Post Office wouldn't have accepted this, right?
(I'm hoping for a UK-specific answer, but happy to hear answers relating to other countries. The subject came up because social media posts are noting that US presidential candidates are sending donation requests with prepaid reply envelopes.)