As dan_nl stated, you can add a private fake repository in package.json. You don't even need name and version for it:
{
...,
"repository": {
"private": true
}
}
Update: This feature is undocumented and might not work. Choose the following option.
Better still: Set the private flag directly. This way npm doesn't ask for a README file either:
{
"name": ...,
"description": ...,
"version": ...,
"private": true
}