An LN2 sprayer could work, but it would be impractical and less effective than a more conventional compound. I can maybe see an open tank of LN2 as a hazard in some manufacturing plant (don't fall in!), but even that would be kinda implausible. Otherwise, I just don't see it being used as a weapon in any practical way. The biggest hazard from a portable tank of the stuff would probably be suffocation if the LN2 was used to fill a small room (cell, bedroom, storage closet...); CO2 would work just as well, and be cheaper and easier to get.
Dunno if you looked this up, but here's some info from Wiki:
QUOTE (http://en.wikipedia.org/wiki/Liquid_nitrogen)
At atmospheric pressure, liquid nitrogen boils at 77K (-196C; -321°F) and is a cryogenic fluid which can cause rapid freezing on contact with living tissue, which may lead to frostbite. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at 77 K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks.
Liquid nitrogen can easily be converted to the solid by placing it in a vacuum chamber pumped by a rotary vacuum pump.[2] Liquid nitrogen freezes at 63 K (−210 °C; −346 °F). Despite its reputation, liquid nitrogen's efficiency as a coolant is limited by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect, known as the Leidenfrost effect, applies to any liquid in contact with an object significantly hotter than its boiling point. More rapid cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen than into liquid nitrogen alone.
Since the liquid to gas expansion ratio of nitrogen is 1:694 at 20C, a tremendous amount of force can be generated if liquid nitrogen is rapidly vaporized. In an incident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were malfunctioning and later sealed. As a result of the subsequent pressure buildup, the tank failed catastrophically and exploded. The force of the explosion was sufficient to propel the tank through the ceiling immediately above it.[6]
Because of its extremely low temperature, careless handling of liquid nitrogen may result in cold burns.
As liquid nitrogen evaporates it will reduce the oxygen concentration in the air and might act as an asphyxiant, especially in confined spaces. Nitrogen is odorless, colorless and tasteless, and may produce asphyxia without any sensation or prior warning.[7] A laboratory assistant died in Scotland in 1999, apparently from asphyxiation, possibly caused by liquid nitrogen spilled in a basement storage room.[8]