public class CIMVoxelUniqueValueRenderer : CIMVoxelRenderer, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

//var voxelLayer = ... ;
//Must be on the QueuedTask.Run()

var variable = voxelLayer.GetVariableProfiles().First();
var renderer = variable.Renderer;
if (variable.DataType == VoxelVariableDataType.Continuous)
{
    //Renderer will be stretch
    var stretchRenderer = renderer as CIMVoxelStretchRenderer;
    //access the renderer

}
else //VoxelVariableDataType.Discrete
{
    //Renderer will be unique value
    var uvr = renderer as CIMVoxelUniqueValueRenderer;
    //access the renderer
}

//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);