Prevent tab into WPF DataGrid

Question

I would like to prevent tabbing into a read-only Microsoft data grid in WPF. I set IsTabStop to False, but I can still tab into the data grid. Any ideas?

<WpfToolkit:DataGrid
    IsReadOnly="True"
    IsTabStop="False"
    ItemsSource="{Binding Path=GridData, Mode=OneWay}" >
</WpfToolkit:DataGrid>