wpf DataGridTemplateColumn.CellTemplate how to enable/disable checkboxes

I have a DataGrid with DataGridTemplateColumn.CellTemplate defined like this :

<DataGridTemplateColumn>
     <DataGridTemplateColumn.Header>
          <CheckBox ToolTip="Select all items" 
            IsChecked="{Binding IsSelected}" Name="chkSelectAll" Checked="AllItem_Checked"
            Unchecked="UnCheckAll_UnChecked" IsHitTestVisible="{Binding Path=IsSelected}"/>
     </DataGridTemplateColumn.Header>
     <DataGridTemplateColumn.CellTemplate>                                        
        <DataTemplate>
             <CheckBox HorizontalAlignment="Center" Checked="Item_Checked" Unchecked="Item_UnChecked"
                IsChecked="{Binding IsSelected}" IsHitTestVisible="{Binding Path=IsSelected}" />                                        
       </DataTemplate>
     </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>

The grid has a binding with a list of objects.

I want that, for each object of ItemsSource, if IsSelected attribut is TRUE the CHECKBOX BECOME IsReadOnly (can't be checked or uncheck.

I don't know how to perform this; I tried Binding IsHitTestVisible property to IsSelected ItemsSource Objects attribut but it's not working.

I googled about and found some topics about Multidatatriggers but can't understand how it's work.

Can someone help me ? I'm new with WPF.

Thank you.