How to disable a databound ListBox item based on a property value?

You can use ItemContainerStyle:

<ListBox>
  <ListBox.ItemContainerStyle>
    <Style TargetType="{x:Type ListBoxItem}">
      <Style.Triggers>
        <DataTrigger Binding="{Binding YourPropertyName}" Value="False">
          <Setter Property="IsEnabled" Value="False"/>
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </ListBox.ItemContainerStyle>
</ListBox>