I found a way: 

Dim element As Infragistics.Win.OptionSetOptionButtonUIElement

element = DirectCast(UltraOptionSet1.UIElement.ChildElements(0).ChildElements(xxx), Infragistics.Win.OptionSetOptionButtonUIElement)

With element
    .Enabled = False
    .Invalidate()
End With

Change xxx for the index of the ValueListItem you want to disable. Item still clickable though 🙁