Something like this. I haven´t tested this though but it should do the trick. Don´t know if 200 pixels is enough or too much. Just make sure your layout has room to scroll.
Maybe it´s also possible to use system > set canvas size and basically shave off 200 pixels when an ad is shown but I´m not sure.
My image isn´t showing... EDIT: Of course now it shows, duh :V
On banner shown > Set scroll Y to -200
On banner hidden > Set scroll Y to 0