Users deleting tasks ???

Hi,
I started using zoho projects and am very pleased with it, however I need some help : I created some milestones and tasks on my project, then assigned tasks to some "employees". After some testing, one of the project users with a level of "employee" deleted a complete milestone with task lists and tasks, everything ! Is it possible for a regular base user to delete tasks or even change due dates on my global project milestones ???? I think this is wrong, is there anyway I can prevent this ?? Thank you.