Oto link do standardowych stylów i szablonów combobox: http://msdn.microsoft.com/en-us/library/ms752094.aspx.
Powinieneś móc dodać styl, podobny do poniższego, który sprawia ComboBox płaski (może potrzebować trochę szczypanie):
<Style x:Key="CustomComboBox" TargetType="{x:Type ComboBox}">
<Setter Property="HorizontalAlignment" Value="Stretch"/>
<Setter Property="VerticalAlignment" Value="Top"/>
<Setter Property="MinWidth" Value="60"/>
<Setter Property="UIElement.SnapsToDevicePixels" Value="True"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
<Setter Property="TextElement.Foreground" Value="Black"/>
<Setter Property="FrameworkElement.FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="BorderBrush" Value="Black"/>
<Setter Property="BorderThickness" Value="1" />
<Setter Property="Background" Value="White" />
</Style>