One of the worst triggers to buy more is criticism of spending.
Being told, 'youve got loads of them already', 'thats the same colour as ones you've already got', ' you don't need that', 'why are you buying more stuff?', etc.
Negative remarks act as a trigger to behave in defiance and a determination to go ahead and buy! You don't want to admit that the person may be right even though you know they are, as admitting to the problem is distressing and brings feelings of shame, guilt, embarrassment, humiliation etc, all feelings that you want to avoid. It's easier to block those out and buy it.
You feel in control when you buy.
My own opinion is that it is a manifestation of wanting to be in control. Many years ago I had eating disorders and that was all about being in control.
My parents, especially my father were quite strict and I grew up with him being an authoritarian parent and I never made choices, my life was decided for me. Of course I did rebel in small ways, wearing the sensible shoes they bought me whilst smuggling out the stiletto's in my bag to change into later!
Controlling what I ate made me feel that I had some control in my life. As I got older and started a family I was preoccupied with my children and my eating disorder abated.
However, a new issue developed and that was in the form of spending. I bought far too much for my children and then as they grew up I spent more and more on my home, yarn as crochet is my hobby and on myself, clothes, jewellery, make up and beauty products and things for my dogs.
I've been fortunate as I have had the money to do so and have not got into debt or applied for credit etc.
My point being that if you have a partner who is compulsively buying and they carry on when you ask them tomatoe, it's not that they want to be disrespectful to you or come across as being arrogant or selfish, they are trying to protect themselves from the feelings that I mentioned above.
Like any addiction, it cannot just be stopped overnight but it certainly can be controlled especially with support and kindness because if you feel good about yourself the urge to spend lessens.
Please understand op, the person overspending has usually developed this compulsion because they feel bad about themselves. Whilst you are frustrated, angry and upset, it does not help and may even make the situation worse when you make those feelings clear to your partner.
I suggest that you act with kindness in helping her get to the root cause of the problem, whether it be with professional help or not and I expect it will be something that happened or goes back to her childhood.
Best wishes to you in dealing with this complex problem.