You can remove a volunteer from your organization's page SEE Deleting a Volunteer, which will get rid of all their hours from your page. However to delete hours from a specific volunteer is a one-by-one process.
Go to Organizer Dashboard >> Volunteers >> List of Volunteers:
Find the volunteer whose hours you would like to delete (use search funtion - magnifying glass icon - or click on the underlined name if it appears in the list) and click on the underlined name. >> click Edit >> Delete >> Save