How to lock task on the planning?
You can lock your tasks by two different ways:
Forbid creation and modification of tasks prior to the current date
From PlanningPME, go to the menu “Tools” -> “Options…”

On the tab « General », the following window appears:

Ticking on this box, past tasks to the current date will not be allowed to be modified.
Task locking
You can fix some tasks in order no one to modify them without your agreement.
First of all, to have access to this option, you must enter the schedule with users rights.

Then, double clicking on the locked task, you will see the name of the user who locked the task. Here, David.

Only the person responsible for the locking or the administrator will be authorized to unlock the task selecting “unlock”.

Isn't tasks management easier now?