Sometimes you just have to be rude. If the don't take the first "no, thank you - good luck" as an answer (my stock reply to anyone asking for anything) they get another "no" and hung up on. because they don't let up. They do seem to mistake being polite as a sign of weakness. My thing isn't usually that I don't have money, but rather I do not like being cold called. I donate when I want to donate, on my terms or not at all. There are so many causes out there nowdays that it is simply impossible to give to everyone without yourself going bankrupt.
Misty you took the words right outta my mouth!
I once had a charity ask me "what, you can't afford $10?" when I said finances were tight! First of all it's none of their business, second it's just plain rude.