After done with an AWS Elastic Beanstalk instance, tried to stop and then terminate it to regain some spaces back. To my surprise, no matter how I tried, AWS always automatically relaunched new instance.
It baffles me for a few days, and did some research. Some articles pointed to auto-scale configuration, and I then tried the suggested approach, and failed.
Armed with the hint of auto-scale configuration, I browsed around, and find these two sections, Auto-Scaling Group and Launch Configuration. Deleted related settings in these two sections, and my AWS Elastic Beanstalk instance terminated.