You should be able to do this either in the deployments status -> all hosts view:
Or within the edit process group details you can switch off monitoring for that process group on specific hosts:
Does that help?
If it has been shut down on the host and you can't see it, just select a timespan (in the top bar) that includes the time when the IIS service was still running. Then you will see the process group.
I think it will just vanish automatically after 72 hours after shutdown as it is effectively removed from SmartScape.