We have business accountholders that are requesting debit cards for individuals and do not want them to be authorized signers on the account. I have resisted this but am not an expert on Visa rules by any means. My primary concern is how would we prevent the business from claiming unauthorized use on virtually any transaction they chose?

The only person who can claim unauthorized use of a card per VISA rules is the cardholder. We require an authorized signer of the business account to sign the debit card application for the employee granting them the authority to have the card. The responsibility then lies with the business to notify the bank if the employee leaves the company and the card needs to be cancelled.
VISA's zero liability states that, "Financial institutions may exclude from the Zero Liability policy a transaction made by a person authorized to transact business on the account and/or a transaction made by a cardholder that exceeds the authority given by the account owner."

So the only risk to the bank is if the employee loses the card which is no greater or lesser risk than any other issued card.