In your Manage Contract Jobs screen, you'll see that some jobs have a button marked Close Job:
If this button is showing that means that the job's contract has been fully invoiced and all invoices are paid by the client.
If the button still shows Invoice Now, it means that there is still an outstanding balance on the contract:
If neither button shows, it means that the contract has been fully invoiced, but not all invoices have been paid:
Jobs can be closed at any time, regardless of contract and invoicing status. For more information on how to do this, please click here.