Webpage should not be accessible till popup is closed/submit

Question

Hi, I used the following code to open the popup on in a silverlight  XAML page, but i can access the web page other links when the pop up is open, the webpage should not be accessible till the popup is closed , could any one please say hot to achieve it.
My XAML code is :  ( and i am open popup by setting popMessage.IsOpen=true in xaml.cs )
<Popup x:Name="popMessage">
<Grid x:Name="theBack" Background="#80000000">
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition Height="Auto" />
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Border BorderBrush="#7F4895DE" BorderThickness="1,1,1,1" Width="400"
Height="200" Padding="10,10,10,10" CornerRadius="25,25,25,25"
Grid.Column="1" Grid.Row="1">
<Border.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF000000" />
<GradientStop Color="#CC4895DE" Offset="0.844" />
<GradientStop Color="#FF346592" Offset="0.393" />
<GradientStop Color="#FFFFFFFF" Offset="1" />
</LinearGradientBrush>
</Border.Background>
<StackPanel Height="Auto" Width="Auto" Orientation="Vertical" >
<TextBlock x:Name="txtAddr1" Height="143" Width="Auto"
Text="Message Goes Here" FontFamily="Trebuchet MS" FontSize="24"
HorizontalAlignment="Center" VerticalAlignment="Center"
Foreground="#7FFFFFFF" TextWrapping="Wrap" TextAlignment="Center" />
<Button x:Name="btnClosePopup" Height="30" Width="80" Content="Close" />
</StackPanel>
</Border>
</Grid>
</Popup>