I operate a wedding venue and had sent a contract and Invoice out to a client. They never executed the contract and do not respond to my emails or made payment. So I made that date available again, now have another couple interested in booking. I want to send them a contract, but don't want the first couple sending theirs back after the fact and then have a conflict. If a document, such as a contract is past the 'due date' and has not been submitted, I'd like it to be locked/voided so the guest can no longer sign/submit it (unless I extend the 'due date'). For Invoices, to 'void' it so a client can't make payment, but this would need the functionality to accept payments after the due date since some may just make a late payment. Users should also be able to mark a document as void, regardless of the due date.