Even with the mainsheet pinned in you will only slow down a little, plus the boat will try to slew to windward (on some boats this effect can be quite considerable). The best way to slow down is to reduce sail area, say half drop the mainsail, the only way to stop quickly (or at all in some conditions) is to round up and face the wind.
As a beginner i wouldn't recommend a downwind approach under sail unless in a
dinghy, which makes an ideal learning boat.