This is not something that can be turned off. This setting in Administration->Project Management->Timesheet Options is used to set the default for new uers when they create their timesheet. If this is unchecked, then when a user creates their first timesheet, this option will also be unchecked. The user can override the default by checking the option and when the user goes to create their next timesheet, the value will be checked. See KB: KB000124960
for more details on this setting.