You can reject an applicant by opening the applicants form, and in the top right corner click the “Reject applicant”.
You then get 8 reason you have to select to say why you reject the candidate (application, position closed, education, experience, skill set, competency, licensure, failed assessment) and a comment to why they are rejected.
In the job you get an overview of all applicants you rejected and why they were rejected.
Malin Martnes 
How can I unreject an applicant if a mistake was made?
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