Although the new administrative assistant appears to be the problem, she is a symptom. The problem is the president. He has either given her too much power, or she as grabbed it. Either way, the fact that he is willing to do nothing about this matter only means that it is going to get worse unless you take further action.
You said this administrative assistant blocks your access to the president, but you are allowing this to happen. There must be times early or late in the day when she is not around, and you should take advantage of them.
You and your fellow managers should meet again with the president and give him specific examples in which her blockages and directives have created problems, wasted time, and interfered with productivity. Present specific suggestions to redefine her role, and make sure the president sees that these steps will not call for additional time or responsibilities for him.
In addition, if you are simply rolling over when his assistant gives you directives, that should stop. If she has a good suggestion, you should implement it, just as you would for any other employee. And if she issues an order, let her know she is out of order.
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