Tasks Pro™ is designed to be used in a collaborative environment, to help people work together rather than than an enforcement tool. That said, there are a few ways this can be done.

Generally, this is the easiest:

1. Set your assignee permissions so that they can not edit tasks/notes that are not owned by them.
2. Create a small task tree for each assigned task (using a template is a good idea here) that contains a Master Task, owned by the task assigner that contains a full description of the task, and a Sub-Task, assigned to the lower level user.

Your assignee can add new sub-notes/tasks and can complete their task (which will in turn complete the master task), but they cannot alter the master task.