To delete an Auto Scaling Group, you can use the cmdlet Remove-ASAutoScalingGroup. If the Auto Scaling Group has EC2 instances or scaling activities in progress, you must specify the Force switch in order for deletion to succeed. If the Auto Scaling Group has policies, deleting the Auto Scaling Group deletes these policies, the underlying alarm actions, and any alarm that no longer has an associated action.
PS C:\> Remove-ASAutoScalingGroup -AutoScalingGroupName WebAppASG
PS C:\> Remove-ASAutoScalingGroup -AutoScalingGroupName WebAppASG -ForceDelete $true -Force
The first cmdlet did not run to success, as the Auto Scaling Group has EC2 instances running. The second cmdlet had a ForceDelete switch, which terminated all the underlying EC2 instances and then removed the ASG.