In order to prevent planned project times being exceeded for customer projects, you have the option of restricting the creation of tasks for a project for corresponding user groups with the user rights for task creation. The users in this user group can then only create tasks for contacts or opportunities, but not for projects.
You can find this setting under Administration – user management – user and rights managementby checking the box next to “Create contact and opportunity tasks” option.
Learn more about user rights here.