Add this code

 

Private Sub DockingPaneManager_Action(ByVal Action As XtremeDockingPane.DockingPaneAction, ByVal Pane As XtremeDockingPane.IPane, ByVal Container As XtremeDockingPane.IPaneActionContainer, Cancel As Boolean)
On Error Resume Next
    If (Action = PaneActionDocking) Then
        If (Container.Container.Type <> PaneTypeTabbedContainer) And _
        (Container.Direction = DockTopOf Or Container.Direction = DockBottomOf Or Container.Direction = DockRightOf) Then
            Cancel = True
        End If
    End If
End Sub

 

For a working example, get the Docking Pane sample MDIPanes, and edit the OnDockingPaneNotify to have the following code:

 

LRESULT CChildFrame::OnDockingPaneNotify(WPARAM wParam, LPARAM lParam)
{
 if (wParam == XTP_DPN_ACTION)
 {
  XTP_DOCKINGPANE_ACTION *pAction = (XTP_DOCKINGPANE_ACTION *)lParam;
  if ( pAction->action == xtpPaneActionDocking )
  {
   // restrict to docking only left and right
   XTPDockingPaneDirection eDir = pAction->dockDirection;
   if ( eDir == xtpPaneDockTop || eDir == xtpPaneDockBottom )
    pAction->bCancel = true;
  }
 }
 return FALSE;
}