You can either mark an applicant as inactive or you can archive them which will move them to your customer archive.
To mark an applicant as inactive first find the applicant, go the edit page and untick the 'Active Applicant' tick box.
To archive a customer, follow the steps below:
You can Restore an applicant at any time. Simply search for an applicant using Find Customer and make sure 'Include archived' is ticked. Then edit the applicant and click the 'Restore' button.