Interesting question (+1) - I'm not aware of any option to hide the auto scale metrics in the Amazon CloudWatch web interface and the respective PutMetricAlarm API action doesn't feature a related option either.
While your use case is sound, the AWS team usually follows the Minimum viable product approach when implementing a new service initially, and despite steadily adding additional features later on, they still tend to obey the Pareto principle when choosing worthwhile features, so I doubt that your understandable request will be implemented anytime soon.