I imagine the only way he could stop the marriage would be to send a crack team of operatives deep into Aquitaine to abduct Eleanor, smuggle her back to Louis, and force her I to a marriage. I doubt he has the pull or the money to actually even able to consider this, but wealthy heiresses faced this problem the minute their fathers died all the time and many of them were sadly forced into these marriages. If someone truly cunning and cutthroat was in Louis' ear, perhaps he orders an audacious mission like this. Then again, it might just be easier to assassinate Geoffrey, but Geoffrey had brothers to take his place and still marry Eleanor....unless Geoffrey's brothers are treacherous.