Working with AWS Security Groups

To remove security groups, you first determine the Group Ids. You can do that by using one of the following commands:
aws ec2 describe-security-groups | grep GroupId
or
ec2-describe-group | grep sg-
once you list all security groups you can use one of the following commands to remove them:
aws ec2 delete-security-group --group-id
or
ec2-delete-group <sg-id>

Best Practice for Launching a Hadoop cluster in EC2

Launch hadoop instance through management console. Although if you don't have access you can use aws cli for that. The following command will list all AMIs that have hadoop installed:
ec2-describe-images -x all | grep hadoop
The second command allows you to launch an instance from the AMI that you previously selected:
ec2-run-instances <ami id for example: ami-0111f768> -t <instance size ie: m3.medium>

The default security groups for the VPC (see this article) cannot be deleted.