代码非标准，仅供参考。

using System;

using System.Collections.Generic;

using System.Linq;

using System.Windows.Controls;

using Telerik.Windows.Controls;

namespace slTest

{

    public partial class MainPage : UserControl

    {

        public MainPage()

        {

            InitializeComponent();

            this.combo.SelectionChanging += combo_SelectionChanging;

        }

        void combo_SelectionChanging(object sender, SelectionChangingEventArgs e)

        {

            e.Cancel = this.isCancel.IsChecked.Value;

        }

    }

    public class Combo : Telerik.Windows.Controls.RadComboBox

    {

        public Combo()

        {

            base.SelectionChanged += Combo_SelectionChanged;

        }

        private object _orginalItem { get; set; }

        void Combo_SelectionChanged(object sender, Telerik.Windows.Controls.SelectionChangedEventArgs e)

        {

            if (e.AddedItems.Count != 0)

            {

                if (this._orginalItem == e.AddedItems[0]) return;

                if (SelectionChanging != null)

                {

                    var addedItems = new List<Object>();

                    if (e.AddedItems.Count != 0)

                        addedItems.AddRange(e.AddedItems.Cast<Object>());

                    var removedItems = new List<Object>();

                    if (e.RemovedItems.Count != 0)

                        removedItems.AddRange(e.RemovedItems.Cast<Object>());

                    var cancelEventArgs = new SelectionChangingEventArgs(addedItems, removedItems, true);

                    SelectionChanging(sender, cancelEventArgs);

                    if (cancelEventArgs.Cancel)//模拟cancel为true

                    {

                        var removedItem = e.RemovedItems.Count != 0 ? e.RemovedItems[0] : null;

                        this._orginalItem = removedItem;

                        this.SelectedItem = removedItem;

                    }

                }

            }

        }

        public event SelectionChangingEventHandler SelectionChanging;

    }

    public delegate void SelectionChangingEventHandler(object sender, SelectionChangingEventArgs e);

}

<UserControl x:Class="slTest.MainPage"

             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

             xmlns:local ="clr-namespace:slTest"

             xmlns:telerik="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Input"

             mc:Ignorable="d"

             d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">

        <StackPanel Width="300" HorizontalAlignment="Left" Margin="5">

            <local:Combo x:Name="combo">

                <telerik:RadComboBoxItem>1</telerik:RadComboBoxItem>

                <telerik:RadComboBoxItem>2</telerik:RadComboBoxItem>

                <telerik:RadComboBoxItem>3</telerik:RadComboBoxItem>

            </local:Combo>

            <TextBox x:Name="selectedValue"></TextBox>

            <CheckBox x:Name="isCancel"></CheckBox>

            <Button Click="Button_Click">Button</Button>

        </StackPanel>

    </Grid>

</UserControl>