Get one of your other drunken female friends to hit her back.
I've been hit by a few girls when i've been on the far side of soberness. Getting right up close and bellowing 'FUCK...YOU' into their faces always seems to work for me. Then again, that works on just about everyone
Pour your drink over her.
Edit: You don't need to buy her a drink. If you'd spilled it everything over her, sure, THEN you'd be obligated, but merely spilling someones drink requires a quick apology and then a hasty retreat.