AN AMERICAN BOYCOTT of Libyan oil is an important gesture. But it's necessary to recognize that, in present circumstances, it is no more than a gesture. The Reagan administration ought to proceed only if it is ready to maintain this boycott in times of shortage as well as in the current surplus of oil.

For the present, Libya, still trying to enforce inordinately high prices, will have great difficulty selling its oil regardless of boycotts. Also regardless of boycotts, the United States will have no difficulty at all finding the oil elsewhere to meet its diminished import requirements. But, as everybody ought to understand by this time, events can reverse the market without warning and suddenly give all the advantages to the sellers. That's no reason to abandon the plan for a boycott. But the country needs to be aware that the boycott will not necessarily continue to be as costless and easy as it now appears.

Libya is the rare case in which economic sanctions are clearly worth invoking. Often, economic sanctions are poor weapons--ineffectual, injuring innocent bystanders more gravely than the politicians and policies that are the targets. But in this case, the general population--a small population, in a country with great wealth--is not going to suffer from the boycott. Neither is any other country likely to be touched by it. The purpose here would not be to starve the country or to use economic duress as a means of punishing it for the reckless and violent forays abroad that have characterized Col. Muammar Qaddafi's rule.

The point of the exercise is the expression of moral disapproval--in the Wilsonian tradition. Because Col. Qaddafi has used the enormous Libyan oil revenues to finance assassination squads throughout Europe and in this country, and to build up his armed forces to threaten his unfortunate neighbors in Africa, the United States is well justified in ending its inadvertent contributions to those depredations. If others wish to buy the oil, knowing how the revenues will be used, that is their affair--although it might be noted that the Europeans have been reducing their imports of Libyan oil significantly.

No doubt the Libyans can find other customers, if they choose to cut their prices further. No doubt they can find other engineers and technicians to run their oil fields, replacing the Americans who have been withdrawn. The United States is not trying to snuff out their economy. It is simply declaring that Libya, under Col. Qaddafi, has placed itself beyond the pale, and it now proposes to deliver that message by isolating Libya from it. The United States broke diplomatic relations some time ago, but in the Libyan case it is the commercial relations that count. The right response is the refusal to buy from them.