Easy, just do a search in the ebay "Help" ,menu.
You have to do it all the time if you want to end a listing early (ex: a car that sold locally).
ebay will not let you end an auction with active bids, so you have to cancel the bids before ending the auction.
It can get kind of hairy if you have a bunch of people bidding and you try doing it in the last few minutes.
In your case I assume that you only want to cancel one bid? After you do so if it were me I would also block that buyer
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How do I cancel a buyers bid? 
