The Reagan administration formally banned further imports of Libyan oil yesterday and accused Libyan ruler Col. Muammar Qaddafi of "a particularly outrageous" attempt last November to kill possibly hundreds of U.S. officials and their families by planting plastic explosives in stereo speakers at an American club in Sudan.

State Department officials claimed that "we know" such explosive devices were prepared in a neighboring country by Libyan intelligence officials, who then supervised the loading of the speakers on a plane bound for Khartoum, the Sudanese capital. Fortunately, the officials claimed, the Libyan plot was discovered and the shipment stopped in another country before reaching the Sudan.

Officials stressed that the action against Libya, news of which leaked out last month, was not the result of any particular action by Qaddafi's regime. Rather, said State Department spokesman Dean Fischer, it was "in response to a continuing pattern of Libyan activity which violates accepted international norms of behavior."

At the White House, however, officials said the most significant factor in the president's decision to halt all oil imports was that those imports constituted a major inconsistency in American foreign policy. "It did not make policy sense," as one official put it, "to criticize terrorism and on the other hand to provide as much as $10 billion a year to his Qaddafi's coffers to fund this kind of thing."

Qaddafi's reported involvement with international terrorism and subversion, and his increasing military adventurism in northern Africa, have been a major preoccupation of the Reagan administration. But the United States continued to buy Libyan oil and as recently as last fall both the president and senior aides were saying an embargo would not be workable unless it were worldwide, because Libya could sell elsewhere whatever the United States did not buy.

Now, however, the administration believes an embargo is necessary, anyway, to show U.S. resolve and to focus attention on Libyan policies even though it may have, as yesterday's official statement said, "only limited economic impact on Libya." The administration also believes the embargo will give the United States greater credibility with countries that feel threatened by Libya, such as Egypt, Sudan and Saudi Arabia.

The new presidential order, which was tentatively decided on last December but not announced until after consultations with Congress and foreign governments, prohibits imports of Libyan crude oil and requires validating licenses for all U.S. exports to Libya except for food, other agricultural products, medicine and medical supplies. The order also lays out a general policy of denying licenses for the export to Libya of U.S. oil and gas technology and other items that could involve national security or high technology that could be used for military purposes.

Officials acknowledge that the current world oil glut makes this an easier time to impose an embargo but deny that that was the reason for it. According to officials who briefed reporters yesterday under rules that do not allow identifying them, U.S. oil imports from Libya had already fallen to about 150,000 barrels daily, leaving them at about one-fourth of their former peak and only about 3 percent of U.S. energy imports. But even these reduced imports at today's reduced world prices are equivalent to about $2 billion a year, or 25 percent of Libya's current oil revenues.

The administration did seek support from allies in Europe and Japan but White House officials say what it got was verbal, not substantive. State Department officials said they "don't expect that our allies are going to join us." The major allied users of Libyan oil are Greece, Turkey and Italy, officials said, and possibly West Germany. The most enthusiastic backer of the administration action, officials said, is Egyptian President Hosni Mubarak, who recently talked with President Reagan about the Libyan problem.

Although Libya has pulled its troops out of Chad, and alleged threats of Libyan "hit squads" in this country have subsided, officials here believe these shifts are only "tactical," in part because Qaddafi seeks to become president of the Organization of African Unity.

The administration delayed imposition of the embargo partly to allow Americans to leave Libya. Last year there were still more than 2,000 there. Now, the number is down to 350 and these are said to be "virtually indigenous Libyans" who probably will not leave and are not in much, if any, danger.

Aside from efforts to subvert countries such as Sudan, Somalia and Oman, where State Department officials say the U.S. has vital interests, these officials said yesterday that Qaddafi is also providing support for "radical" governments in Central America and the Caribbean, including Nicaragua. The officials declined to provide evidence of that.