How many more innocent civilians will have to be killed before Francois Mitterrand, Margaret Thatcher, Helmut Kohl and other European leaders decide to put a sense of morality and dignity ahead of their economic trade with Libya?

The killing of 15 innocents and the wounding of another 110 travelers in random shooting at the Rome and Vienna airports two days after Christmas apparently was not enough to stir the Europeans to action. Yet this was merely the latest in a long list of horrible acts of violence attributed to Abu Nidal, a terrorist whose chief sponsor appears to be Muammar Qaddafi.

Next to oil, terror appears to be Libya's main export. Some U.S. officials have recommended military retaliation against Libya, but a divided Reagan administration hasn't been able -- yet -- to take this step, conscious of the chain reaction it could set off. But why should more blood have to be spilled in front of ticket counters before European politicians agree to join in international economic sanctions against Libya, which sells them 80 percent of its oil? In return, most of these countries sell Qaddafi a whole range of goods -- including critically needed oil-field equipment.

According to the OECD, the Common Market countries in 1984 imported $7.2 billion worth of goods from Libya (mostly oil) and sold them $3.5 billion in miscellaneous items. The British actually had a small surplus of trade with Libya in 1984 that they'd like to protect. But Italy and West Germany had the biggest two-way trade with Libya.

Clearly, European politicians are more desirous of sustaining the volume of trade than they are conscious of the need to respond to America's greater concern to find a way of dealing with terrorism. To be sure, Europeans make the argument that economic sanctions are not effective -- that if they don't sell to Qaddafi, someone else will. This may be true. The real question, however, is a moral one. There is a time to come forward and do what is right, whether or not the price is the loss of business to someone else.

The United States in the past five years has let its commerce with Libya dwindle to almost zero -- while warning its nationals to leave the country -- and is now trying to squeeze out what's left. It's time for the Europeans to join in, to declare Libya a pariah among nations and Qaddafi an international offender who should be excommunicated.

Is there to be no response at all from Europe when Qaddafi threatens to drag the rest of the world into war? Or must we wait until tourist traffic through Heathrow and Charles de Gaulle has fallen off enough to wake up the business communities in those countries? Qaddafi asserted in a staged-for-television press conference that there are no Palestinian training camps operating in Libya, "but if they need, I will give them -- I am not afraid." This disavowal of a link with guerrilla bands can be taken for what it's worth.

Israeli Prime Minister Shimon Peres, who doesn't pull any punches, said that no one should be surprised to find that Qaddafi is, among other things, a liar. More diplomatically, Sen. Patrick J. Leahy (D-Vt.), vice chairman of the Senate Intelligence Committee, said he found Qaddafi's denial "incredible."

Qaddafi may not own up to his alleged relationship with Abu Nidal, but he was quick to praise the terror wreaked on the Rome and Vienna airports. "Heroic" was what Libyan officials called it, while Qaddafi threatened to unleash World War III if anyone dared to retaliate against his country.

Morality aside, it's time to play hardball with Libya. Now that there is a glut in oil supplies and prices are declining, a buyers' cartel against the purchase of Libyan oil would be even more effective today than when first recommended in 1981 by then secretary of state Alexander Haig. The Europeans rejected the idea then, Claude Cheysson of France saying that Haig had only "convinced himself." They are still opposed, and will have to be dragged along, kicking and screaming.

Haig said in a telephone interview that he shares European skepticism about the effectiveness of economic sanctions, and doubts that Qaddafi, "as heinous as he is, is at the centerpiece" of the problem. But Haig agrees that Qaddafi supplies the resources, if not the manpower, for terrorism, which he attributes to Syria and Shiite fundamentalists.

Short of dropping bombs directly on Libya, the only way for the civilized world to deal with Qaddafi (that is, if the civilized world has a sense of dignity left) is to cut off any vestige of civil dealings. As President Reagan argues, that means economic sanctions to deprive Libya of its key source of income -- from sales of oil -- and to make its economy suffer by not selling to it the consumer and capital goods items that it needs.

If Qaddafi continues to provide a strategic base for international terrorism, stronger international action will be needed. But the first response ought to be economic, along with diplomatic isolation. And that can work only if Europe screws up its courage.