I would just tell the sales ppl that you aren't interested and to ask them to stop up selling cause you not interested. The fact is, Costco as a whole has a very low turnover rate compared to most retail outlets. We told our cashier (we say our, wife and I,) go to the same people all the time. People can get used to faces. We have been members for about 4 years and love them.
As too how they make their money, anything Kirkland is their product. Its well known they make decent money off this, not just member sales.