My experience with this in PC World. I bought a small item, about £12, and was immediately offered an extended warranty. I declined before she got the first sentence out. Of course, that wasn't the end of it, "Just let me tell you about it"
"No point, I'm not buying an extended warranty on a £12 item, no matter how good it is" I said.
At this point, I've started walking from the display to the checkout, and now she's following me, "You must let me tell you, i'll get in trouble otherwise"
"Really not interested" I said at this point, giving up all pretences of being polite.
By now i'm queing at the checkout, so i'm a captive audience, and she starts again. This time i literally give her the 'talk to the hand' hold my hand up between her face and mine, and looking away. I hear her finish with "so shall I arrange that for you?" before I just chuckle pay and walk off.