So I am assuming that the host was decommissioned? Never the less, you can edit your hosts by going to Settings>Monitoring>Monitoring Overview>Hosts
After a few days of the host reporting as offline, Dynatrace will no longer alert on it too.
If you dont have the option on the list below, you might not be able to. but after 3 days it should drop out. We had this issue as well where the server team did not communicate decommissions and as a result, we would be alerted that the hosts were no longer responding or being monitored.