I think that every single person who posts on this message board has gone through exactly what you describe. There is no easy way to 'fire' a volunteer but here are some things that you can do.
First, explain to her right up front that there is a new sheriff in town. Do this along with your new financial advisor. If she won't meet with you, then you go to her. Apprise her of what is expected, update schedule, forms that need to be filled out etc. Tell her that this is prootcol and that EVERYONE is expected to abide by it. Inform her that if she thinks that there will be a problem with any of these requirements, then by all means, you totally understand if she chooses not to participate. You need to be diplomatic but firm. I like the expression 'quiet intimidation'. You don't have to be loud or brash to get your point across clearly. And lastly is follow through. Assign someone on your executive board to be her point person. Let that officer know exactly what is expected and the time frame. If she fails to report in, then simply remove her as per your previous discussion.
I would also add all the formalities of $ reporting to your by-laws.
Never...NEVER allow a volunteer control alot of money and refuse the checks and balances. That is a recipe for disaster.
With every position, paid or volunteer, there are performance requirements. Because of her extreme aversion to communication and cooperation, you really have no choice but to relieve her of her duties. Will she treat this action with anger and hostility towards you? Of course. However, the entire committee, and ultimately the entire organization, will be more productive and will be exposed to less risk if you do what needs to be done.
I have a volunteer of one of our fundraising committees who is extremely uncooperative. She is in charge of a lot of money and yet has no interest in sharing information, giving udpates on the committee, etc. The past president didn't deal with it and now it is in my lap. Our new financial advisor wants to meet with us to discuss new reporting issues and she doesn't even want to do that. This is only the tip of the iceberg. Her committee doesn't like working with her and I have a continual headache. Can you "fire" a volunteer?