public SpatialIndexDescription( 
   Index spatialIndex,
   FeatureClassDescription featureClassDescription
)

public void RemoveSpatialIndex(SchemaBuilder schemaBuilder, FeatureClassDefinition featureClassDefinition)
{
    // Create a spatial description  
    SpatialIndexDescription spatialIndexDescription = new SpatialIndexDescription(new FeatureClassDescription(featureClassDefinition));

    // Enqueue the DDL operation to remove index 
    schemaBuilder.Delete(spatialIndexDescription);

    // Execute the delete index operation
    bool isDeleteIndexSuccess = schemaBuilder.Build();
}