Good news: After much trial and error, I have now succeeded in getting nPower to add me to their "Marketing Suppression Database".
Bad news: So far they've sent me four letters telling me that they're no longer going to contact me again. The latest letter tells me they've tried to phone me several times to tell me they're not going to contact me, and can I instead phone them within 5 working days otherwise they may not be able to fulfil my request. I ring them and say "this is weirdly like a nightmare, isn't it?"
Next time I am dating, I shall behave more like nPower.
"Hi Ustvlad, I understand you said you would call me rather than me calling you, but I am just calling you to see if that is still the case. I'm sorry - would you just mind holding for a second while I call your details up? That's lovely *blast of Leona Lewis*. Now, let me just see - do you have a reference number to hand Ustvlad? No? Don't worry, we'll just set you up on the system. Bear with me, this won't take a moment. *blast of Jedward singing Jerusalem*...."