Restrict changes in task assigned
Enable Security for a assigned task can only be modified by the owner of that task/project. Once a task has been assigned to a member of the group, only the creator of the task can modify the assigned member of the group and dates
Andrei Stolbov commented
Also assigned person should not be able to delete assigned task without owner confirmation.
This could be an optional setting for the whole plan. For example, you can assign a task to some team member and owner of the group has full control over deadlines set, completion, etc.