It probably depends on the yeast you're using. Unless you were improvising, I don't know how you could have ended up with potential alcohol that high, as that would require a LOT of sugar.
If you DO manage to get the fermentation to stop now... it will probably be too sweet to drink. Better to let it finish.
And depending on your yeast, it will probably stop before it hits that alcohol content anyway. What yeast did you use?
I suppose you could try watering it down or using it as a mixer?