There could be situations where you would want approvers to only approve timesheets and expense sheets, however you may not want them to generate a timesheet or expense sheet.
This can be achieved by creating a new permission or editing an existing permission. Make sure the 'Timesheet' and 'Expenses' categories within the permission are disabled. Once this is done you need to assign the new permission if created to the user.
Note: If there are multiple permissions assigned to the user and if any one permission has the timesheet and expense categories enabled the user will then be able to generate timesheets and/or expenses.