To exclude a specific timesheet entry from invoicing:

  # Go to *Timesheets > Timesheet > All Timesheets*
  # On the Timesheet list view, check *Non-billable* for specific timesheet entries

or:

  # Go to *Project > All Tasks*, ans open a specific Task form
  # On the Task form, *Timesheets* tab, check *Non-billable* for specific timesheet entries

To have access to this *Non-billable* field, the user must be added to the 
*Exclude timesheets from sale order* security group.
