Every other day....

"That's called tech support, I'm a programmer." (Add "but I can look at it" if you feel like helping)
"Great, do you also have funding?" (If you actually like the idea, offer to fund them with your work, and tell them the percentage you think it is worth. Preferably somewhere between 80 and 99)
"Can a mechanic build a fighter jet?"

These answers are good because they relate to the core issue: the ever underestimated complexity of software development.

@Lor-inc For the second point, I once answered like you suggested and got the answer:

Oh, come on, we are friends. You can do it for free, right?

😄

@Lensflare Letting them explain the idea and giving a quick (and generous) estimate of how many hours it takes should do the trick. Hours are a unit of work everybody understands. If they still don't give up, they're just dicks and you might want to reconsider your friendships.

@Lor-inc will give it a try next time! 😄