I just reply that I am so far behind on the ones I want to make that I will never catch up, so the possibility is not good that I would get to one for them. That stops them in their tracks.
If someone kept pushing (and they never have), then I would give them the "do you have any clue what you are asking me to do?" speech. I'm pretty comfortable just saying no. My time is mine and I don't give it away unless I want to. No apologies for that.