There is nothing you can do apart from block the bidder so they can't come back and do it again.
Ebay don't let bidders cancel if they just change their mind.
They have 3 choices:
Bid wrong amount
Seller changed description (which can't happen because once an item has a bid on, the seller is not actually allowed to change the description)
Can't contact seller.
So, if a bidder just has a change of heart, decides they don't need or can't afford the item or simply buys elsewhere, they have to choose one of the official reasons.
It is better for you to have a bid cancellation than an unpaid item, a request for cancellation or a return.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)