I did it
because I also needed that in one of my projects!
my solution is platform independant and accurate responsive.
there's an event handler wich is triggered when a form is moved :
OnChangeBounds
double-click on it at design time, in the events tab of object inspector...
as my form is screen centered and i set Position to poScreenCenter I wrote :
procedure TMainForm.FormChangeBounds(Sender: TObject);
begin
MoveToDefaultPosition;
end;
and each time you try to move it, the form comes back to the center of the screen.
If you need a special position you just can do something like
procedure TMainForm.FormChangeBounds(Sender: TObject);
begin
BeginUpdateBounds;
Left := 120;
Top := 80;
EndUpdateBounds;
end;