To deactivate a custom scoring system on a task you need to go to the task page (you can access it from the Assigned tasks tab in the group where you assigned the task to) and click on the More actions menu, there you will find the Edit task option.
Once editing the task you can go to the I want to customize my students' grades section and on the dropdown select Default score system. The custom score system won't be used on this task anymore.
We hope we have cleared your doubts, otherwise, please let us know.
