For deleting the attached EBS volume first you need to unmount to the directory and then detach it from the instance then try to delete then you can delete the volume. If the ebs is created from instance then you need to terminate the instance. For AMI you need to terminate the instance and then try to de-register the AMI and snapshot can be deleted only after the volumes got deleted. Try the above steps that will help you to delete these resources.
For more information, check this AWS Tutorial.