I'm with czar... I don't even respond. Just delete. I find the hardest ones to deal with are people that I know that really believe in their cause but think I should too. And when I don't, all of a sudden I am the plague... whatever....
It is hard for them to grasp that by donating a cake, it costs me 2x's what they are asking. Because I only have so much time and can only take a certain number of orders, I have to eliminate a paying order to take the free one. It's money out of my pocket for their ingredients and then lost income on the order I couldn't take. I don't even bother to explain it... but I KNOW that it wouldn't be giving with a smile unless I WANTED to do it and BELIEVED in the cause I was donating to. I HAVE donated a free cake. But it's not often.
My Dad always said, "NEVER give away for free what you do for a living." If you are in business to make a living, you have to remember that this is your livelihood. It's not a non-profit. If it is a hobby, it's different. But when it's your paycheck... well... I just hit delete.