There's this company that kept trying to send me spam fax, but I don't have a fax, so I'd pickup the phone and just hear beeps. I decided to call it, and got a fax machine. Hooked up a fax machine so I could get it ready and just had to hit receive next time I get the call. Managed to receive it. Was some kind of lumber store spamming all over. I googled it and lot of people were getting this.
I made a word doc that said STOP IT in white text on a black background, copied the page a couple hundred times, then used a free fax service to send it. Also sent them threating emails.
They seem to have stopped.
I've always been under the impression this was illegal, but I called the cops the originating city and they said there's nothing they could do.