It's possible, but you would need to retemplate it to achieve perfection. You can get most of the way there by overriding a system parameter as follows:
<ComboBox xmlns:sys="clr-namespace:System;assembly=mscorlib">
<ComboBox.Resources>
<sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">0</sys:Double>
</ComboBox.Resources>
<ComboBoxItem>One</ComboBoxItem>
<ComboBoxItem>Two</ComboBoxItem>
<ComboBoxItem>Three</ComboBoxItem>
</ComboBox>
However, it isn't perfect because the focus rectangle still assumes there is a drop-down button present.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…