Customized close document via keyboard

Hello,

I found this thread:

How to close document window via keyboard

and my question is related to that fiunctionality (to stop that).
I have the following "outer" DockSite:

                <views:CustomDockSite x:Name="dockSite" CanDocumentWindowsFloat="True" FloatingWindowTitle="xxx"
                          WindowsClosing="OnDockSiteWindowsClosing" WindowActivated="OnDockSiteWindowActivated" WindowDeactivated="OnDockSiteWindowDeactivated"
                          AreNewTabsInsertedBeforeExistingTabs="False" AreDocumentWindowsDestroyedOnClose="False" WindowsDragging="OnDockSiteWindowsDragging"
                          DocumentItemContainerStyle="{StaticResource DockingWindowStyle}" DocumentItemsSource="{Binding DocumentViewsService.DocumentViewsOpened}">
						  
						  <!--More code here-->
						  
                    <docking:Workspace x:Name="workspace">
                        <views:CustomTabbedMdiHost/>
                    </docking:Workspace>
						  
						  <!--More code here-->
						  
		</views:CustomDockSite>

and "inner" DockSite:

         <docking:DockSite x:Name="styleDocumentDockSite"
                          WindowsClosing="OnStyleDocumentDockSiteWindowsClosing" WindowActivated="OnStyleDocumentDockSiteWindowActivated" WindowDeactivated="OnStyleDocumentDockSiteWindowDeactivated"
                          AreNewTabsInsertedBeforeExistingTabs="False" AreDocumentWindowsDestroyedOnClose="True"
                          DocumentItemContainerStyle="{StaticResource DockingWindowStyle}" DocumentItemsSource="{Binding Tabs, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" >
            <docking:Workspace>
                <docking:TabbedMdiHost HasTabImages="True" TabStripPlacement="Top"/>
            </docking:Workspace>
        </docking:DockSite>

Please note mainly the DocumentItemsSource for the outer docksite. It is a collection of viewmodels and the view for such viewmodel is the inner docksite. It means we have here docksite contained within other docksite. Is there a way how to stop the default behaviour of Ctrl+F4? Becasue I want to use that for closing the views inside the inner docksite (the command to be bound to the gesture (input binding) is already implemented on my side) but it seems to be somehow blocked by the default behavior.
Btw, when the inner views are active pressing Ctrl+F4 has no effect.

Thanks for your response.