A judge ruled Thursday that former Orange County Treasurer Robert L. Citron had legal authority to make the high-risk bond investments that caused the 1994 bankruptcy debacle, a ruling that severely limits the county's chance to recover more losses. In lawsuits against former brokers and other professionals, the county contended the investments violated the California Constitution and state laws. That contention was thrown out by U.S. District Judge Gary L. Taylor. County lawyers said they probably will appeal the ruling, which negates suits against more than a dozen Wall Street firms that did business with Citron. The decision has no effect on the $739 million in legal settlements already agreed to by a number of major defendants, notably $437 million from the county's former chief investment house, Merrill Lynch & Co.