Trap the Windows WM_WINDOWPOSCHANGING message and "or" the flags of the WindowPos structure passed in the message's lparam parameter with the predefined constants SWP_NOMOVE and SWP_NOSIZE. Example:

type
  TForm  = class(TForm)
  private
    { Private declarations }
    procedure WMPosChange(var Message: TWMWINDOWPOSCHANGING);
       message WM_WINDOWPOSCHANGING;
  public
    { Public declarations }
  end;
var
  Form : TForm ;
implementation
{$R *.DFM}
procedure Tform .WMPosChange(var Message: TWMWINDOWPOSCHANGING);
begin
  PWindowPos(TMessage(Message).lParam).Flags :=
     PWindowPos(TMessage(Message).lParam).Flags or
     SWP_NOMOVE or
     SWP_NOSIZE;
end;