I donate to United Way in my payroll deduct every year. I aslo donated to CHEO and other health related charity. I have the same feeling here that once one donated, then he/she should do it forever. Once one donated to one charity, then he/she should do it for the rest.
Well, I think that we should still handle it properly for those unsolicited calls and should not leave others any impression that we are mean people after we have contributed generously. Here is what I did for the calls during the past few days.
"I have already donated these amount to X,Y,Z and etc. This is the budget of my family for this year and I believe that we are generous enough. You may find other people or family who haven't done so..."
Normally, the callers would say "Thank you very much!" And they may be even volunteers themselves and giving them a hard time is not the best way.