"preventable" implies that somehow the people or leaders of the time knew of it before hand and did nothing to stop it, which is simply not true. now, if some of the conditions that helped caused and feed the revolution were different, then yes, it may have not happened, but prevention is not really the right term.
What you would really want to look at are the causes (such as the lack of food due to harvest failures in the parisian region, the huge public discontent at the queen for being austrian, the radically changing ideas concerning the role of the common man in governance, the increased tax burden on the poor to fundpast and current foreign wars, the perks given to the nobility that increased the burden on the vast majority of people, the intense regional division, the breakdown of the systeme d'etapes, etc) and how many of those would need to be changed in order for the french revolution to have not occurred. however, since we can't go back and pull apart history like we can a cell, all we can do is speculate as to what would have happened if some of these facts had been different.
you must also remember that this was the first major revolution in western europe that came from the people against their own royalty (yes, you can argue what occurred in england, but that was driven by the nobility and mostly resulted in shifting the power of the king to the lords, and america was a colony). to be frank, there has always been discontent amoungst the poor, and there have always been food riots and other such things. but they had never seen nor could they expect the extent of change and revolutionary fervor that would come about from what appeared to be normal grumblings of the peasants and working man. even if there had been signs of discontent, they would be seen in leiu of past experiences, and their importance would not be understood as something more until much too late to "prevent".