You would need to retemplate the GroupOverflowArea. We ship the default xaml as part of the install so you can use the default style for that element from the DefaultStyles directory as the basis for creating your custom style. I would recommend not entirely removing the PART_OverflowMenu since that menu should be available in case there are more groups than can fit in the overflow area or else your end user will not be able to select any groups that don't fit in the overflow area. Instead you may want to just remove the menu items that you don't want to be there (e.g. ShowMoreButtonsMenuItemHeader).