@James Hart , Apologies for the delay in response. Try with this command for removing ghosted NICs,
Get-AzNetworkInterface | where-object { $_.VirtualMachine -eq $null } | Remove-azNetworkInterface
If you want to list the unattached NICs then you can use this command,
Get-AzNetworkInterface | where-object { $_.VirtualMachine -eq $null } | ft name
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