You cannot prevent students from editing posts and replies they create in the discussion tool. However, you can track the history of each post. Start by going to the "Tools" dropdown in the blue navbar and click on "Discussions." Next, navigate to the topic you want to view and click on the caret next to it, then …
Begin by clicking on the "Course Admin" tab in the blue navbar. Then scroll down under "Assessment" and click on "Rubrics" Next, click on the caret next to the rubric you want to adjust and click on "Edit" From here you can adjust the percentage, point value, etc of the rubric.