You can lock the Mark as Hired Button which locks the applicant's hired status and disables the Mark as Hired button after an applicant is hired.
To enable the Lock Hired Button:
1. Go to Your Account/Account Details
2. Scroll down to the Customize & Design section and click the Workflow button
3. Scroll down to the Applicant Hired section and click the toggle switch to the right of Lock Hired Button to ON
To Disable the Lock Hired Button simply click the toggle switch to turn it OFF