That is 1 option, reconfigure the default OSCustomizationNicMapping.
Another option is to remove the default one (I assume it is in position 2 after your New-OSCustomizatioNicMapping cmdlet.
$nicMapping = Get-OSCustomization -Name YourName | Get-OSCustomizationNicMapping | where {$_.Position -eq 2}
Remove-OSCustomizationNicMapping -OSCustomizationNicMapping $nicMapping
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