Charity Harrassment

We support about 8 charities, some on a regular basis and some as and when.  We receive on average 14 begging letters a week and probably 8 phone calls asking us to commit to direct debit.  Just tonight had 2 charity workers knock on the door.  We suspect that our details like thousands of others are on a hit list of people (who regularly give money) which is being passed around the charities.  We have now started to come down hard on the charities that we already support and are still after more.  We tell them we will not increase our giving and if they contact us again we will stop all donations to their charity.  So far we have stopped giving to one charity. Do you have the same problem and if so how do you deal with it