If a crew member leaves before the end of their assigned contracted period, you can terminate their contract through the Contracts page in your Vessel Dashboard.
Simply follow the steps below to terminate an active contract:
Find the crew member you wish to terminate.
Under Actions (top left) select the Remove icon.
Choose the date you wish the contract to be terminated from.
And that's it. You'll now see the contract only show when you select the Terminated : Show in the filter bar top right.
Important to know
Even after you've terminated the contract, you'll still be able to access it via the contracts page, or via the previous crew member tab on the crew details page.