Why would someone do such a thing? What does she think she is going to gain by this behavior? Why would she think she can just order things and expect us to pay for them?
Those are really good questions and unfortunately, I don't know! Perhaps she is feeling left out (doubtful but maybe) she's bipolar and has gone off her medications....
However, since she is no longer authorized to make these orders on behalf of your group, you are not obligated to pay for them. The businesses recieving the orders just need to be notified these items are not officially authorized and that your group will not pay for them.
In the meantime, perhaps you need to put the word out to other vendors, associates or whoever that they may be contacted and to be aware that she no longer is a authorized person on your board.
We've never had to do this in any of our PTAs but we have had to do this with business associates at my former business.
If it can't be stopped this way, another option is a "cease and desist". It's extreme but it will get across in no uncertain terms that needs to stop this behavior or she could be held liable for the costs.