It seems creating a NuGetDefaults.config file in C:\Program Files (x86)\NuGet\Config does the trick:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<!-- defaultPushSource key works like the 'defaultPushSource' key of NuGet.Config files. -->
<!-- This can be used by administrators to prevent accidental publishing of packages to nuget.org. --> <config>
</config>
<!-- Default Package Sources; works like the 'packageSources' section of NuGet.Config files. -->
<!-- This collection cannot be deleted or modified but can be disabled/enabled by users. -->
<packageSources>
</packageSources>
<!-- Default Package Sources that are disabled by default. -->
<!-- Works like the 'disabledPackageSources' section of NuGet.Config files. -->
<!-- Sources cannot be modified or deleted either but can be enabled/disabled by users. -->
<disabledPackageSources>
</disabledPackageSources>
</configuration>