How to clean grid content

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WPF

MVVM

I have the grid as:

<DockPanel Height="280">
          <Border CornerRadius="6" BorderBrush="Gray" removed="LightGray" BorderThickness="2" >
              <ScrollViewer VerticalScrollBarVisibility="Auto">
                  <ItemsControl Height="400" Name="icTodoList" ItemsSource="{Binding Items}">
                      <ItemsControl.ItemTemplate>
                          <DataTemplate>
                              <Grid Name="ToDoList">
                                  <Grid.RowDefinitions>
                                      <RowDefinition Height="*"/>
                                      <RowDefinition Height="*"/>
                                      <RowDefinition Height="*"/>
                                      <RowDefinition Height="*"/>
                                  </Grid.RowDefinitions>
                                  <TextBlock Text="{Binding StartTime, FallbackValue=' '}" Grid.Row="0"/>
                                  <TextBlock Text="{Binding ConnectedTime, FallbackValue=' '}" Grid.Row="1"/>
                                  <TextBlock Text="{Binding DisconnectedTime, FallbackValue=' '}" Grid.Row="2"/>
                                  <TextBlock Text="{Binding DialingResult, FallbackValue=' '}" Grid.Row="3"/>
                              </Grid>
                          </DataTemplate>
                      </ItemsControl.ItemTemplate>
                  </ItemsControl>
              </ScrollViewer>
          </Border>
      </DockPanel>

By clicking a button to start a process, some contents are add to the grid, But next time new stuffs are appended after the old ones. I don't want that; I want to clean up the grid once I click the button, so the layout always show the new contents.

How?

EDIT:
I used MVVM pattern.

public ObservableCollection<Calls> items = new ObservableCollection<Calls>();
   public ObservableCollection<Calls> Items
   {
       get { return items; }
       set
       {
           items = value;
           RaisePropertyChanged();
       }
   }

And in the code behind:

MainViewModel _dataContext;
Calls c = new Calls();
// blah blah
 Dispatcher.BeginInvoke((Action)delegate()
        {
            if (c != null)
                _dataContext.Items.Add(c);
        });