To delete a report owned by someone else you will need to login as that user.
Steps are as follows:-
-Login as Administrator. -Click on Administration. -Go to Users under Employees and Organization. -Click on the User who owns the report. -On the top right hand side click on 'Login as' -Go to 'Reports'. -Click on the required report. -Click on 'Settings'. -Click on 'Delete Report'. (Note that once deleted, reports cannot be recovered again)