i hear the introduction at the moment and politely interrupt with "as much as i would love to support your charity, we simply don't have the money to help at the moment, so i'll let you get off this call and onto another"

i have had one person push it. they got a firm but polite no, have a nice day, and i hung up

i guess working in a call centre (esp as a supervisor taking the calls that have escalated) i'm probably more confident with call control and i don't allow myself to be pushed
i GET that they need to call or they'd get nothing, and i regularly donate what i can, but if i'm rudely pushed by someone, they go to the bottom of the "maybe when we have money" pile.