The state’s highest court Thursday banned the Registry of Motor Vehicles from increasing some sanctions against drivers who acknowledge they were drunk when stopped by police, but whose cases are ultimately dismissed by a judge without a finding of guilt. Writing for the unanimous court, Supreme Judicial Court Justice Margot Botsford said that when Beacon Hill lawmakers toughened the state’s drunken driving law in 2005, they did not change the legal definition of what qualifies as a “conviction’’ of a driver.