CCP Has put a confirmation window on the ‘Delay’ button which you can go back and talk to the mission agent again to ‘undo’, but there is no confirmation window on the Reject Mission button - something that you cannot undo, and have to hope you get the mission again reasonably soon…
so simple suggestion - put a confirmation window on this button. It would of course have the checkbox option of not showing it again, with the reset settings option to undo that checkbox…