You can enable close button on the navigator tabs. Add the IsShowClose attribute in the Navigator definition.

If True, then a close button is displayed allowing the tab to be closed by a user.

<l:NavigatorTabItem 

         x:Name="Tark4ShellNavTab" 

         Header="Tark4"

                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"                 

         IsShowClose="False"

>