You have to draw the line somewhere. For example, let's say a certain charity like cancer research asks you to donate and you send them $20. All is well. Then since they know you are a donater, they send you another appeal. You send them another $20.
Then they send you something for a black tie event that costs $185 per plate. You bite the bullet and accept that. They schedule a few more in the next coming months for at least $185 per pop.
Do you keep on accepting those? You just have to draw the line. Have the personal satisfaction that you did do some help but you are not obligated to give everything out.