Under no circumstances change that feedback.
I wouldn't let that seller chase me off ebay though.
I'd respond to one and only one message. Say you are NOT going to change it under any circumstances, ever, and their harrassment has made you feel threatened and has only served to make you realise you did the right thing in giving them a negative. That what they are doing is against ebay policy. And to leave you alone, please stop the harrassment, because you will not be reading any more messages from them, they will be deleted immediately.
Then after that, ignore and delete. You may find that they give you a few more messages but I think there is a limit on how long they can do that for. I know that sellers & buyers can communicate for a few months after a purchase but once someone has been blocked, once that time runs out, they won't be able to.
I'm not sure what the time limit is, someone else may be able to help you there, I thought it was about 6 months but I don't really know. That's the reason they are still able to message you even though they are now blocked. They won't be able to do it forever, just in the period after the sale.
But I can see your purchase was some time in the last 6 months. So it is over a month ago. Once you send off your reply, they cannot claim they did not know you were upset or felt threatened, they could not claim you had not asked them to stop. That's why i would send it.
You could get onto ebay chat and talk to a rep, ask them to see the message history and ask if they will block that seller from messaging you now.
But if it is really bugging you, you could also just take a breather from ebay for a month or two. Once you come back just do a bulk delete of the messages and by that stage, the seller will probably have stopped.
