The XamTreeItem has an IsEnabled property which would disable the XamTreeItem and the checkbox item.

The other option would be set the CheckBoxStyle of the XamTreeItem and change the IsEnabled on a style object to style only the CheckBox object.

    <UserControl.Resources>

        <Style x:Key="cbStyle" TargetType="CheckBox" >

            <Setter Property="Background" Value="Green"></Setter>

            <Setter Property="BorderThickness" Value="1"></Setter>

            <Setter Property="BorderBrush" Value="Red" ></Setter>

            <Setter Property="IsEnabled" Value="False" ></Setter>

        </Style>

    </UserControl.Resources>

private void Button_Click(object sender, RoutedEventArgs e)

        {

            Style s = this.Resources["cbStyle"] as Style;

           XamWebTreeItem ti = tree.ResolveXamWebTreeItem(tree.Items[1]);

            ti.CheckBoxStyle = s;

           //ti.IsEnabled = !ti.IsEnabled;

        }

However there is a bug where setting this style won't correctly update.  This should be resolved in the next service release and I will have our Developer Support staff create a case for you to be notified when this issue is resolved.