Nagios configuration that only notifies 1x down and then only notifies the recovery

In your host definition, set notification_interval to 0:

define host{
    use                     generic-host
    host_name               example
    address                 example.com
    notification_interval   0
    }

You can see the available configuration options for a host here: http://nagios.sourceforge.net/docs/3_0/objectdefinitions.html#host

You should log into the nagios interface and acknowledge the outage this should stop subsequent alerts, remembering to clear this once service has been restored, alternatively if this is scheduled maintenance, place the service / server in scheduled downtime.

Setting the alert me only once, is not what I would call best practice, as you are telling the system to assume you got the first alert, and not to send subsequent alerts, which is a perfect world would be fine, however this is not always the case such as a failed email send due to temporary smtp outage or temporary sms gateway outage.