I tend to give negative product reviews when harassed by the sellers like this. In my opinion, the review should also be reflective of the company making/selling the product.
If you go to http://www.amazon.com/feedback you can review the sellers instead of the product.
To be fair though, I believe people are much more likely to see a product rating than a seller rating. As evidenced by the number of "unbiased" 5 star reviews.