You can hide the winning bid amount for an auction but editing your Auctioneer settings. Click here to see how to edit your auctioneer settings.
Once you are editing your auctioneer, go to the Main Settings tab and scroll down to the Bidding section. Here you will see a tickbox labelled 'Show Winning Bid'. Untick this to hide the winning bid as soon as the item closes (please note the winning bid value will still show for the winning bidder only).
Once you have made any required changes, click the green Save Auctioneer button at the top of the screen.