Most have a pre-paid envelope with them and it will only cost them money if it is returned to them.
Nearly thirty years ago, my partner and I split up. She left to live elsewhere.
She'd always taken Kay's catalogue. They kept sending it, so I wrote to them saying that Miss X didn't live here any more so would they please take her off their mailing list. Instead, they made up a wife for me (never had one of those) and started sending the catalogues to "Mrs Haywood". After I'd tried several times to tell them that there was no Mrs. Haywood it was clear that they weren't interested.
I was building a house at the time. So one day I took a damaged engineering brick, they're the heavy ones, and I wrapped it up in brown paper. I sellotaped one of their first class return envelopes to it with a note in it saying that if they wanted any more building materials delivered to their desk, like for example a couple of dozen broken roof tiles or half a hundredweight of wet finish plaster, all they had to do was leave my name on their mailing list.
Never heard from them again.