Below is some VBA code to show you how to do this, key Interface is ICodedValueAttributes
Public Sub TurnONOFFCodedDescription()
' Get Document
Dim pMXD As IMxDocument
Set pMXD = ThisDocument
' Get map
Dim pMap As IMap
Set pMap = pMXD.FocusMap
' Get first layer and cast into IGeoFeatureLayer
Dim pLayer As ILayer
Set pLayer = pMap.Layer(0)
Dim pGeoFL As IGeoFeatureLayer
Set pGeoFL = pLayer
' Get annotation layer properties for this layer
Dim pAnnotationPropertiesColl As IAnnotateLayerPropertiesCollection
Set pAnnotationPropertiesColl = pGeoFL.AnnotationProperties
Dim unplacedEle As IElementCollection
Set unplacedEle = New ElementCollection
Dim placedEle As IElementCollection
Set placedEle = New ElementCollection
Dim pAnnotateLayerProperties As IAnnotateLayerProperties
pAnnotationPropertiesColl.QueryItem 0, pAnnotateLayerProperties, placedEle, unplacedEle
' Get label engine
Dim pLabelEngineLayerProperties As ILabelEngineLayerProperties2
Set pLabelEngineLayerProperties = pAnnotateLayerProperties
Dim pAnnotationExpressionEngine As IAnnotationExpressionEngine
Set pAnnotationExpressionEngine = pLabelEngineLayerProperties.ExpressionParser
' Set Coded value to be false (same as unchecking on GUI)
Dim pCodedValueAttributes As ICodedValueAttributes
Set pCodedValueAttributes = pAnnotationExpressionEngine
pCodedValueAttributes.UseCodedValue = False
End Sub