If you don't want the ListBox to provide selection at all, you could use a plain ItemsControl instead of a ListBox:
<ItemsControl ItemsSource="{Binding}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<CheckBox Content="{Binding}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Then you will just have a sequence of CheckBoxes without wrapping them with ListBoxItem controls that will take the keyboard focus.
On the other hand, if you want the ListBox to display selection, then maybe you want a multi-select ListBox where the state of the CheckBox is bound to the selected state of the ListBoxItem. Since clicking the ListBoxItem will check the CheckBox, you could prevent the CheckBox from being focused at all:
<ListBox ItemsSource="{Binding}" SelectionMode="Multiple">
<ListBox.ItemTemplate>
<DataTemplate>
<CheckBox
Content="{Binding}"
Focusable="False"
IsChecked="{Binding IsSelected, RelativeSource=
{RelativeSource AncestorType={x:Type ListBoxItem}}}" />
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
