IME anyway, deflecting actually works on a lot of people, because they understand it to mean "no" or "I'll have to think about it" or something like that, and they don't repeat their demand. So, one might assume/hope the conversation would go like, "Hey, I'll be doing this charity thing, do you want to donate?" "Oh, that's great! I'll take a look at your FB page for it later." "Okay, thanks." The end.
But if experience with this particular person/group shows that deflection only leads to more demands or expectations, saying "no thanks" at the outset might be the better way to go, from now on. Maybe in a very cheerful tone with some beandip, like, "No thanks, but it's great you're doing it! I hope you have a lot of success! What route will you be walking?" Of course some people will cause drama no matter what answer you give, if it isn't exactly the answer they want, so attempts to make "a nice day" might be futile if you're not willing to hand over the checkbook...
Which wouldn't be your
fault at all, but it can be hard to remember that.