I am attempting to clear events for an unmanaged device without deleting this device, is this possible
You should be able to from the Event Monitor.
I can clear all events, however active alerts are still shown under alerts.
With Basic- you need to un-check the box for the alert- Under Configure Basic Alerts-
For an Advanced you would need to Edit the Alert and un-check Enable this Alert box.
The node that has the active alert is unchecked under monitored network objects, the node itself is unmanaged. The alert is still active.