Hi Burgaud,
I have a Suggestion for you, when you are creating the Project select the Header and Go to Basic Data tab, you find 'Time Recordings' select 'Not Allowed' option from the drop down selection, and Save the project.
then you can go with the regular steps like create tasks and assign services and employees.
but select the Time recording not allowed only at the Header level, then your employees will not find the header while they are entering time sheet or under task selection.
Try this out and let me know.
Thanks
Vinay