The answer to your problem is a soft called Active@KillDisk,ive actually just finished using it on a 16Gig Flash drive whos capacity was down to iGig.
Run Active@KillDisk and in the tree on the left pick out your FlashDrive and tick all the appropriate boxes,then go up the top to the button bar and click on Kill its a slow process but let it do its job,it took me just over an hour for the 16Gig.
Once Active @KillDisk has finished all you have to do is reformat the FlashDrive,you may have to use another computer for this for some reason my computer wouldnt recognise the flash drive but once in another box it was easy to reformat the thing,now its back to its full capacity thanks to Active @killDisk
