<step>ActionScript.SetProperty.Enum("Handles.SourceHandle.LockState", "FullyLocked")</step>
<step>Handles.SetLockState(Handles.SourceHandle.LockState)</step>

Setting the property on its own is not enough as it is only used for display purposes and does not update the handle properties when it is changed. Since the SetLockState function requires an Enum type and not a string you need to either create an Enum value using Math.ToEnum or set Handles.SourceHandle.LockState to the correct value and pass that in.