I just tell them that I already have five boxes from my niece (not true) and thank them for the offer. I won’t give the Girl Scouts my cash but I’m not going to lecture the girls. I think some stranger giving them a speech on sin isn’t going to convince them of anything, personally.
My wife struggled with telling the girls no, and I suggested the same thing. We simply tell them we already bought more than we will eat. They thank us, and we don’t by any.