There's always low tech... lock up the printer cable (or its ink cartridges or its toner cartridge).

If it's a printer that has network capability, you could change it so that it's a network printer and then password protect it from the printer's control panel.

As others have said, on the business version of Vista, there are very clear security settings in the printer control panel. That's kind of ironic, since I can see your situation needing the security more frequently than a business needs it. Most businesses don't want security on their printers, so that visiting dignitaries don't have problems when they visit.