When sharing an office with an administrative assistant who does not do much administering or assisting, you have already played one important card by going to her manager. The fact that her manager refuses to even look into the situation indicates that her managerial skills are in the skimpy range.
At the same time, there are still a couple of cards that you may want to play. If you have a solid working relationship with your own manager, you should consider discussing the situation with him or her. Your manager may be better able to prevail on your nasty associate's manager. Another option is for you to have a businesslike conversation with the new administrative assistant yourself. The likelihood of her changing her ways as a result of anything you might say is slim, but it may be worth a try if she can spare a few moments between personal calls.
If she is truly as difficult as you described, it should just be a matter of time before complaints from customers and other employees start to roll in. When this occurs, either her manager is going to have to take action to deal with her, or senior management may have to take action to deal with both of them.
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