Disabling text selection in DocumentViewer

<Style TargetType="{x:Type ScrollViewer}"  x:Key="CustomScrollPresenter">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ScrollViewer}">
                <Grid Background="{TemplateBinding Panel.Background}">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="Auto" />
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="*" />
                        <RowDefinition Height="Auto" />
                    </Grid.RowDefinitions>
                    <Rectangle Grid.Column="1" Grid.Row="1" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" />
                    <ScrollContentPresenter
                        PreviewMouseLeftButtonDown="ScrollContentPresenter_PreviewMouseLeftButtonDown"
                        Grid.Column="0"
                        Grid.Row="0"
                        Margin="{TemplateBinding Control.Padding}"
                        Content="{TemplateBinding ContentControl.Content}"
                        ContentTemplate="{TemplateBinding ContentControl.ContentTemplate}"
                        CanContentScroll="{TemplateBinding ScrollViewer.CanContentScroll}" />
                    <ScrollBar
                        x:Name="PART_VerticalScrollBar"
                        Grid.Column="1"
                               Grid.Row="0"
                               Minimum="0"
                               Maximum="{TemplateBinding ScrollViewer.ScrollableHeight}"
                               ViewportSize="{TemplateBinding ScrollViewer.ViewportHeight}"
                               Value="{Binding Path=VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                               Visibility="{TemplateBinding ScrollViewer.ComputedVerticalScrollBarVisibility}"
                               Cursor="Arrow" AutomationProperties.AutomationId="VerticalScrollBar" />
                    <ScrollBar
                        x:Name="PART_HorizontalScrollBar"
                        Orientation="Horizontal" Grid.Column="0" Grid.Row="1" Minimum="0"
                               Maximum="{TemplateBinding ScrollViewer.ScrollableWidth}" ViewportSize="{TemplateBinding ScrollViewer.ViewportWidth}" Value="{Binding Path=HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource Mode=TemplatedParent}}" Visibility="{TemplateBinding ScrollViewer.ComputedHorizontalScrollBarVisibility}" Cursor="Arrow" AutomationProperties.AutomationId="HorizontalScrollBar" />

                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style
      x:Key="MyDVStyleExtend"
      BasedOn="{StaticResource {x:Type DocumentViewer}}"
      TargetType="{x:Type DocumentViewer}">

      <Setter Property="Template">
       <Setter.Value>

          <ControlTemplate TargetType="DocumentViewer">
                        <Border BorderThickness="2,2,2,2"
                    BorderBrush="SlateBlue" Focusable="False">
              <Grid Background="{StaticResource GridBackground}"
                KeyboardNavigation.TabNavigation="Local">
                <Grid.ColumnDefinitions>
                  <ColumnDefinition Width ="*"/>
                </Grid.ColumnDefinitions>

                <ScrollViewer Style="{StaticResource CustomScrollPresenter}"  Grid.Column ="0"
                  CanContentScroll="True"
                  HorizontalScrollBarVisibility="Auto"
                  x:Name="PART_ContentHost"
                  IsTabStop="True"/>

              </Grid>
            </Border>
          </ControlTemplate>
        </Setter.Value>
      </Setter>

    </Style>

private void ScrollContentPresenter_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        e.Handled = true;
    }