You could always try the same tactic I have used in the past:
woman phoned cold called me on the phone to try and get me to switch phone suppliers, I bluntly told her 'I'm sorry I don't have a phone line' the silence was deafening before she hung up, presumably after her head exploding from a logical paradox.
so next time the try and sell you something:
'I see you don't have double glazing' – Thats right! these are forcefields I stole from the Enterprise, neat huh?
'I can save you money on your gas bill' – That's ok, we're all steam powered here thanks
'I can save you money on your electricity bill' – Thanks for the offer but my son goes to Hogwarts and your feeble 'electricity' seems a bit of a let down compared to the stuff he can do
etc etc. I find it best if you try and sound like you genuinely believe every word of nonsense you come out with, it's even more fun when they try to argue or get you to admit you're joking….turn it into entertainment and it doesn't seem as bad.