Is there a way to delete a separate document in a batch on the Web Verification Station?
It is not possible to delete the document in a batch from the Batches view. The "Delete" button is only here for deleting a whole batch, so it will always be grayed out when opening the batch and selecting a document.
Instead, you can delete a document after you get the task in the Verification view. Simply select the document from the list and delete it: