I used to click "Get Support" and the report a violation option and explain why the feedback was in violation. That way took days to get it removed, now when I use the opton where you put the order # in and request feedback removal, it removes it instantly. Every time I've used it it says the feedback was in violation and has been removed as soon as I submit it. I don't mind not explaining why it's in violation if they are going to remove it right away everytime.
depends what its for if it has anything that looks like a product review in amazons automated system will remove it.
If its late delivery and you have (any) tracking number showing it (or any other item ) delivered on time it will get removed to
Click Help > Contact Us > Customer Order > Fill in the order number, then you can have the option to submit remove request for the negative feedback, I just did it yesterday, but for a neutral feedback one (Still wondering if it was a mistake or not)
Feedback removal 
) delivered on time it will get removed to
