The only easy way to do this would be by exporting your contacts to a raw text file then using something like Excel to edit out an entire column before re-importing into Now Contact. Doing this might not be a good solution depending on your data however, as it would cause the loss of things like keywords, links between contacts and events, category colors and styles, and might merge all notes into a single note field. If you want further instructions on how exactly to perform such a data export/import, send me a PM and I'll send more detailed steps.
Sean Boiarski
Product Manager, Now Software