WILLIAMSBURG -- Neither judicial robe nor senatorial toga can lure former U.S. Sen. William B. Spong Jr. away from his unfinished mission as head of the Marshall-Wythe School of Law. Spong made that abundantly plain as he began to tell friends that he would not be available for either of two high offices to which his name had been linked by speculation in recent weeks. Answering questions last week from a pair of visiting newsmen whom he has known for many years, Spong effectively removed himself from the field of prospective candidates for the Virginia Supreme Court as well as the U.S. Senate in 1982. A straw poll of Democratic State Central Committee members one week ago indicated Spong was clearly favored for senator by these active part...