Here's an idea that might curb these requests - ask them if you could make a small donation to the organization they are selling for. I mean SMALL - like $5.00. Ask them for the IRS letter showing that the organization they are representing a charitable organization, and that your donation would be tax deductible. There's a little more info here:
charitable organizations
Now - most organizations have some sort of "prize structure," and if you order from your niece or nephew, you'll help them get a prize. If you just give them a check, they personally get nothing, albeit your donation will help their organization. Eventually, my guess is they'll lose interest in you as a customer pretty quick.
Plus - if you take the schedule A deductions, you'll be able to deduct for 100% of the donations you have made.
Good Luck! I specifically seek out a couple of people for fundraiser items I like (my niece's HS band does a great one every year, and I love Boy Scout popcorn too).
We try to limit the fundraisers our DD does. I don't care who she sells GS cookies to - people do not have to buy them. But, she did a fundraiser for band at the beginning of the school year, and right after that product came in, the student council did a magazine fundraiser. We REFUSED to let her sell to the same people, actually, my DH, DD and I each picked out 1 magazine, and that was what she did for that fundraiser. (To be honest, I would have preferred to have just written a check to the student council, but then DD wouldn't have received the credit she needed to attend the student council MEGA Social.
