Suggested approach is to use conflicted node merge option available as part of Settings-> Asset and Inventory Management-> Node Operation ->Conflicted Systems tab and select Conflict option which best suits to understand it is same machine, click on the machine and retain the node of interest using different merge type. By doing this, another node gets deleted. Please note that if this node which is to be deleted is linked to asset, then recommended approach is to use manual merge option to retain the asset data with machine which will be retained. Please refer http://www.sapphireims.com/help/managed_and_unmanaged_node.htm Conflicted Systems option for more details.
There is another way to do handle this and following is the way however if asset is linked to inventory of node going to be deleted, then this is not recommended approach -
Agentless device can be deleted with soft delete option which is available in Settings-> Node Operation-> Select scope as Managed Device and click on button Delete. Please note that this will not delete the node permanently from SapphireIMS.
If permanent delete is needed, you can delete from database and for which you may take help from SapphireIMS support. However, you need to make sure that this device IP is removed from discovery rule to avoid rediscovering the node.