This SCP prevents users or roles in any affected account from running any of the CloudWatch commands that could delete or change your dashboards or alarms.

 
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": [
                "cloudwatch:DeleteAlarms",
                "cloudwatch:DeleteDashboards",
                "cloudwatch:DisableAlarmActions",
                "cloudwatch:PutDashboard",
                "cloudwatch:PutMetricAlarm",
                "cloudwatch:SetAlarmState"
            ],
            "Resource": "*",
            "Effect": "Deny"
        }
    ]
}
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