public CIMVoxelColorUniqueValue[] Classes {get; set;}

//Get the variable profile whose renderer will be changed
var variable = voxelLayer.SelectedVariableProfile;

//Check DataType
if (variable.DataType != VoxelVariableDataType.Discrete)
    return;//must be Discrete to have a UV Renderer...

var renderer = variable.Renderer as CIMVoxelUniqueValueRenderer;

//A CIMVoxelUniqueValueRenderer consists of a collection of
//CIMVoxelColorUniqueValue classes - one per discrete value
//in the associated variable profile value array

//Get the first class
var classes = renderer.Classes.ToList();
var unique_value_class = classes.First();
//Set its visibility off
unique_value_class.Visible = false;

//Apply the change to the renderer
renderer.Classes = classes.ToArray();
//apply the changes
variable.SetRenderer(renderer);