what is the cost effective way to stop the fermentation process in rice wines in order to preserve its sweetness? pasteurization is too expensive and it doesnt seem to really stop the fermentation as the wine turns sour after some months like a wine vinegar.

with out stopping the fermentation process the wine is only good for consumption in less than a month after it is bottled negating its market potential...

Killing yeast in most normal situations needs either Heat or Acid. Proper pasteurization is the normal way to go, unless you are planning to fortify your wine later and are going to add an acid for some reason.

"Most sake is then pasteurized once. This is done by heating it quickly by passing it through a pipe immersed in hot water. This process kills off bacteria and deactivates enzymes that would likely adverse flavor and color later on. Sake that is not pasteurized is called namazake, and maintains a certain freshness of flavor, although it must be kept refrigerated to protect it. "

The most commonly used method is using potassium sorbate. It's commonly found in a product called "wine conditioner" which is a combination of potassium sorbate (to kill the fermentation) and dextrose (for added sweetness).