Tire magnate Mark C. Bloome was fined $850 and his driver's license was revoked after he pleaded no contest to a reduced charge in an incident in which he allegedly struck a Los Angeles police officer with his car. The 84-year-old Bloome was also placed on three years' probation and was ordered by Municipal Court Commissioner Norman P. Tarle to perform 500 hours of community service or pay $2,000 to the police memorial fund. Bloome, on his way to the Hillcrest Country Club last March, allegedly disregarded a roadblock set up by police because of a nearby rally. He was accused of driving through traffic cones and striking the officer, who suffered two broken vertebrae. In exchange for his no-contest plea to a charge involving driving over flares, misdemeanor charges of disobeying a traffic officer and hit-run driving were dropped by the city attorney's office.