The Virginia Board of Corrections rebuffed new corrections director Allyn R. Sielaff today by putting off major changes he announced just last week in the operation of the state's troubled Mecklenburg prison.

The board, meeting for nearly three hours in closed session here with Sielaff, effectively scuttled for now moves Sielaff made without consulting the board.

Sielaff's actions sought to eliminate a controversial behavior-modification program for inmates and alter prisoner assignments to the Southside prison, which has been plagued by unrest and the escape last May of six death row inmates.

"I think he realized a mistake was made, possibly in procedure," said Board Chairman J.W. (Billy) Williams of Charlottesville after the meeting. "He recognized completely that the board sets policy." Williams said Sielaff would present his proposed changes to the board at its January and February meetings.

Sielaff, who has been on the job only since Thanksgiving and is Gov. Charles S. Robb's third corrections director, declined comment today and referred all questions to Williams. The corrections director had previously called objections to his plans a "tempest in a teapot."

Williams called today's unprecedented meeting last week after Sielaff said Mecklenburg's behavior program was a failure and suggested that the concept of putting all of the state's disruptive maximum security prisoners at the prison was not effective.

Sielaff's announcement, made without consulting the board or Robb, surprised and angered some board members who are trying to exert more control over the state's prison system. The board was changed by the legislature from an advisory board to a policy making board in 1983, but it had not asserted its new powers until the May escape.

State Sen. Joseph V. Gartlan Jr. (D-Fairfax), whose committee oversees prison issues, said he was "glad to see the board is asserting itself . . . " Gartlan had criticized Sielaff for implementing policy changes without consulting the board.

"The director recommends, the board adopts policy, and then the director implements it," Gartlan said. "Mr. Sielaff's announcement of a week ago kind of reversed that procedure."

"I think we are definitely on track on what needs to be done in the months ahead," said board member Andrew P. Miller of Alexandria, a former state attorney general who had called Sielaff's actions "a serious mistake."

Miller and others noted that a special committee of the board had recommended that the behavior program, which uses rewards and punishments to control inmates, be improved rather than dropped.

But a spokesman for the American Civil Liberties Union, which repeatedly has won court suits against the state over its operation of Mecklenburg and had praised Sielaff's actions, criticized the board today.

"It's really bizarre that a bunch of bureaucrats second-guess an experienced corrections official," said Alvin Bronstein, director of the ACLU's National Prison Project. "They've never exerted any authority before . . . the first progressive, forward-sounding thing in Virginia, and they're putting it on hold."

The ACLU recently went back into federal court charging that the state is not living up to a 1983 court-approved agreement on Mecklenburg's operation.

The board, citing its intention under the Freedom of Information Act to discuss private personnel and potential litigation issues, went into secret executive session yesterday despite objections from several reporters. The reporters contended that Williams and others had publicly said the board would discuss policy decisions, which generally are not exempt from the FOI restrictions.

An official statement released after the meeting noted that any major changes proposed by Sielaff or the board ultimately will have to be reviewed by U.S. District Court Judge Robert R. Merhige Jr., who as recently as October sharply criticized the state for its handling of problems at Mecklenburg.

The state, which has been on the losing side on prison issues taken to court, has been forced to pay about $300,000 in legal costs incurred by the ACLU as well as thousands of dollars for the state's own lawyers.