Well you can't force her to pay, and if you have the money it's probably not worth making enemies over it. However, I wouldn't now feel any qualms about using her milk in the fridge and such, it's not like she has a leg to stand on if she complains.
If it becomes a regular thing, I would report it to the people in charge of your hall, and probably request a flat change. If it happens more than twice, I would think it's not worth even trying to be friends, and if she won't pay up, just use whatever food of hers is available until you next planned to go shopping.
However, if it's a lot of alchohol (like £50 or more) I would kick up more of a fuss and in future keep it all in your room.