You can delete a candidate but we make it difficult. First, only users in the Manager role can delete. To delete a candidate, go to the Organization Setup menu from the pull-out sidebar. Select the Investigators tab, then open the Investigator to which the candidate is assigned. Then select the Candidates tab on this form and find the candidate you wish to delete. Any candidate on this list can be deleted.
There is no way to restore a candidate once deleted. If you just want the candidate out of view, set their status to Inactive.