It's been a long time since I last sold anything on eBay, but if I remember correctly if you go to the option to cancel the sale, citing "Problem With Buyer's Address" or "The Buyer Has Requested Something Not Offered In The Listing" as the reason why then you should be able to cancel the sale. eBay may well have changed the page layout since I last sold anything on eBay, but from what I remember from reading other recent posts I seem to recall that the option to do so is currently on the right hand side of the page. If I've got this wrong, however, then I'm sure that somebody with more recent experience of selling on eBay will be along at some point to provide some more accurate advice relating to cancelling transactions.
If you intend to relist the item for sale again on eBay then make sure that you add the buyer's User ID to your Blocked Bidders List before relisting the item for sale once the transaction has been cancelled, as that way the same buyer will be unable to purchase the item a second time and cause you another load of problems. It's also worth ensuring that the "Don't Allow Blocked Buyers To Contact Me" option is ticked on your Seller Preferences page, as if it is then anybody who is on your Blocked Bidders List will be unable to harass you with unwanted messages.
Once the "Don't Allow Blocked Buyers To Contact Me" option is activated on your account it will remain active forever unless for some reason you change your mind and decide to deactivate it. Therefore you won't need to activate that option every time you add another buyer to your Blocked Bidders List.
