Comment by idopmstuff

In that case I think it helps to acknowledge that you understand their incentives/motivations and be even clearer than "no" - salespeople are trained that if you hear no the first ten times you ask and yes on the eleventh, that's still a sale.

e.g. "Look, I recognize that it's your job to try to sell me a warranty, but there's absolutely no chance that I'm going to buy a warranty, so let's focus on the product."

If that doesn't work, then you go straight at their incentives - "Listen, like I said, I'm not going to buy a warranty, and this is becoming distracting. If we can't drop the warranty talk, I'm going to guy buy from somewhere else." They want to sell the warranty, but if they know that's not possible then their next incentive is not to have to explain to their boss why they had a customer who spent a significant amount of time with them but didn't close.