Every page of a TPageControl is an instance of TTabSheet.
It has a property named TabVisible. If you set this in the
object inspector to false, you will have the classic notebook
view, without tabs:
// disable all tabs
for i := 0 to PageControl1.Pages.Count-1 do
PageControl1.Pages[i].TabVisible := false;
// force a specific page
PageControl1.ActivePage := PageControl1.Pages[1];