why does aws EC2 instance automatically starts after manually stopping it?

I have a AWS EC2 instance. To avoid more billing , iam manually selecting the instance and stopping it. However in sometime i automatically starts back thereby adding to my billing. How do i permanently stop it and start manually only when i want to

Your EC2 instance must be in an auto-scaling group with a number of instance set to 1.

If you don't need it you can just delete this group and your instance will not start automatically.

Note : You can also check Amazon OpsWorks and Amazon CloudFormation.