An important elevator safety mechanism was turned off when an advertising executive was crushed to death trying to take the lift to her office, city investigators found in a report released Monday. A mechanic overrode the mechanism, a safety circuit that normally prevents elevators from moving with their doors open, to enable work on the midtown Manhattan elevator about a half-hour before an elevator did just that and killed Suzanne Hart on Dec. 14, the city Department of Investigation and Department of Buildings found.