procedure TForm1.Button1Click(Sender: TObject);
begin
if dgColumnResize in dbgrid1.Options then
dbgrid1.Options:=dbgrid1.Options-[dgColumnResize];
end;
--------------------------
dbgrid dgColumnResize:
True: (Default). Columns can be resized by dragging the column rulers
in the title area. Resizing changes the corresponding width of the
underlying TField component.