If the seller was that concerned about it he or she could send you a Feedback Revision Request, which would allow you to revise your feedback to neutral or positive if you changed your mind, but you don't have to change your feedback just because the seller is asking you to do so. Therefore, if you have no intention of changing your feedback just ignore the seller's messages and he should hopefully give up and stop asking you to revise your feedback.
Should you wish to go one step further and ensure that the seller's messages don't even get through to you in the first place then if you don't already have it activated set up the option on the Buyer Management page to stop blocked bidders from being able to contact you. You'll have to add the seller's User ID to your Blocked Bidders list to begin with - the link to the page you'll need in order to do this is as follows: https://offer.ebay.co.uk/ws/ebayISAPI.dll?BuyerBlockPreferences. Click on that link and look at the first sub-heading at the top of the Buyer Management page once it loads up on the screen, which is the one that says "Blocked Buyers List". There's a link right beneath that sub-heading which says "Manage Blocked Buyers List." Click on that link and add the seller's User ID to the white box that appears on the screen, then once you've done that click on the Submit button beneath the white box to confirm the updates to your Blocked Bidders List.
Once you've clicked on the Submit button the Blocked Bidders List page should close and go back to the Buyer Management page. Scroll right down to the bottom of the page and tick the box next to the sentence that says "Don't Allow Blocked Buyers To Contact Me." This is just a one-off procedure and you won't have to keep on activating it every time you want to add somebody to your Blocked Buyers List - it will remain active indefinitely unless you deactivate it manually for some reason, even if you haven't sold anything on eBay for months.
Given that you've recently been in a transaction with this seller the blocks will take longer to kick in than they would have done had you never had any dealings with this seller. However, once the blocks take effect he won't be able to contact you.
Should the seller become really determined to contact you and attempt to contact you via the eBay Community Forums by sending you a PM just open the message, click on the button to block all further private messages from that person and from that point onwards the seller in question will have no way whatsoever to contact you via eBay.
