Customer suck posted to

I was in one of my favorite clothing stores today, as they sell some nice youngish plus size clothing, and had dragged my friend (N) to go clothes shopping.
We both selected some items, and went to pay separately. As I was putting down my purchases on the counter, N was looking at a keychain they had sitting out. The cashier asks us both if we have charge cards, and when N says no she dives into the "Sign up now!" shpiel, because anyone who signs up gets a free keychain or lipstick case. She tells us about how it only takes a second and didn't cost anything blah blah blah. N tells her no, she's not interested. That SHOULD be the end of it, right? Nope, she spent MY entire transaction hounding N about the signing up. "Come onnnn, it only takes a second! It doesn't cost anything! What's the harm? Come onnnn!" and N must have said no about ten times. She only shut up as we were leaving.
I understand cashiers have to tell people about their promotions and special deals, but there is no excuse for being that persistant and irritating.