When Orion has no SNMP authority to a node, the node is marked in the database in the 'Nodes' table under 'ObjectSubType' as 'ICMP' (as opposed to 'SNMP'). For myself, I ran the following query which deleted all of those nodes.
Delete from nodes where objectsubtype like 'ICMP'
This should accomplish what you are wanting to do, but please make sure that you are ready to delete these nodes by running select * from nodes where objectsubtype like 'ICMP' FIRST. This will list the nodes that will be removed.
Are these DHCP nodes? When you add them, why not select the check box as Dynamic Node