The User story automatically into completed status when all the tasks in it are completed. Can this be disabled, if yes how?
Yes, it is possible. An Administrator can disable this at the Project level. If you go to Settings > Workspaces & Projects > locate and edit the project you would like to change. Then disable the field labeled "Auto State Updates" This field is located under the Workflow section.
Here's a link to our docs on the topic: https://docs.ca.com/en-us/ca-agile-central/saas/task-state-updates-parent-schedule-state
It might help to understand under what circumstances the state is updated and the agile methodology under which is makes most sense to have it enabled.