Cardinal Miguel Obando y Bravo, Nicaragua's top Roman Catholic prelate, confirmed today that the church's spokesman, Msgr. Bismark Carballo, has been barred by Sandinista authorities from returning to this country.

The ban was the second move in a sharp crackdown by the leftist government against its domestic opposition in response to the U.S. House of Representatives' approval Wednesday of $100 million in military and nonlethal aid for the Nicaraguan rebels, known as contras. On Thursday the government closed La Prensa, the only newspaper not controlled by the state.

Responding to the House vote, President Daniel Ortega said Friday, that "what we have now is war and we are going to respond with war." Over the weekend Sandinista leaders reinforced his point and said that further restrictive actions would be announced.

Carballo, in Miami returning from a trip to France, told reporters late yesterday that he was not permitted to confirm his reservation with Taca International Airlines to Managua. He was told the airline was instructed by the Sandinista government not to allow him to board any flight to Nicaragua.

There was no official statement yesterday or today to clarify the cleric's situation, but comments in recent days by Sandinista leaders suggested he would be blocked indefinitely from coming home.

Carballo, 36, directs the Catholic Radio, which was closed down by the government shortly after a state of emergency limiting certain civil rights was declared Oct. 15. He was also a chief administrator of the Managua archdiocese and a close associate of the cardinal.

Carballo spoke for the generally conservative Roman Catholic bishops on many issues on which they disagreed with the ruling Sandinistas. The authorities, in banning Carballo, with whom they have clashed before, appeared to be striking at the cardinal without incurring the political risks of taking him on directly.

Political observers in the capital said the new actions marked a turning point in the Sandinistas' tolerance of internal dissent, a theme underscored by Ortega when he said in a speech Friday, "We are not going to be so naive as to accept a civic opposition, because that doesn't exist anymore."

Over the weekend Sandinista leaders escalated their bitter rhetoric against political and religious figures whom they accuse of being allied with the United States and the contras.

"The United States has now declared war on us," said Bayardo Arce, one of nine Sandinista commanders who wield decision-making power in Nicaragua, in a speech yesterday. "We cannot allow any lackey of imperialism to travel abroad to say whatever comes into his head seeking support for the contras, then return calmly as if nothing had happened."

Referring to Obando, Ortega asked in his Friday speech before thousands of Sandinista supporters: "Can this government continue to be flexible with those who are direct instruments of the Reagan government's terrorist policies, just because they hold positions in some hierarchy?"

"No!" members of the crowd responded. "Get them out!"

The cardinal, in a guarded response at a mass today, urged the government to "reflect" on the ban, stressing that Carballo is a Nicaraguan citizen. "When he cannot enter his own country, it's a forced exile," Obando said.

Obando celebrated mass in the rural town of Niquinohomo, birthplace of Nicaragua's nationalist hero Augusto Cesar Sandino, from whom the ruling party takes its name. But rather than referring to Sandino, he emphasized that Carballo was born in a village not far from here.

The government has unilaterally exiled other Nicaraguans in the past whom it has associated with the contras. Economist Arturo Cruz, who ran briefly against Ortega as a candidate for president in 1984, was blocked from returning in mid-1985. Cruz went on to become one of three top contra leaders.

The cardinal did not say that he would protest the ban against Carballo, but he called on Catholics to pray for his colleague.

In the past, the Sandinistas' responses to U.S. support for the contras have varied along with congressional backing. After Congress halted all support for the contras in mid-1984, the Sandinistas lifted some restrictions in time for a campaign leading to the November 1984 national elections. Even then, however, the news media remained under censorship and opposition figures were closely watched. The state of emergency declared last October marked the beginning of a government attempt to break up what it considers to be a growing "internal front" organized by the contras inside Nicaragua.

Western diplomats here said the Sandinistas were shocked that the House of Representatives granted President Reagan essentially the package he requested. The first installment, $40 million including military funds, will be available Sept. 1.

Sandinista officials quickly seized the opportunity of a largely negative reaction to the vote in Europe and Latin America to further restrict internal liberties, the diplomats said. They were also aware that their actions would coincide with a decision Friday in favor of Nicaragua in its World Court suit against the United States for its operations in support of the contras.

Besides the Roman Catholic Church hierarchy, other anticipated steps against internal dissent are expected to affect private business groups and traditional political parties that make up the increasingly powerless opposition. But today they were operating normally.