I am not certain, but I have a feeling that unless the person has been deemed to lack capacity, the law takes the view that they are free to spend or waste their money as they wish. One thing you could do, which has been suggested on here before, is to scratch off the security code on her cards, so that she cannot order anything over the phone. If the company already has her card details, maybe you would have to 'lose' it and doctor the replacement.
However, and I do not wish to be alarmist here, but I have recently gone into this sort of thing a lot over an elderly neighbour, who does not even have dementia, who has spent many thousands of pounds on lottery scams. And I have found out that unscrupulous companies, some of which specialise in toiletries and so called 'health' products, sell on the details of their customers, which then go on 'suckers lists', i.e. they are sold on to all sorts of 'lottery' and other scam artists, many based overseas, who have no qualms about relieving the vulnerable, and particularly the vulnerable elderly, of as much of their money as they can. A common ploy with the 'health' products would seem to be that they promise a large cash prize if the person keeps ordering again and again - the prize will of course never materialise - do you know if this is the case with your mother?
If you do not already have a power of attorney in place, do please try to arrange it ASAP. And if you can, check her mail or get it redirected (you would need P of A or she would have to agree) before she is targeted by anyone else. My neighbour who has been completely hooked by the lottery scam was originally targeted by phone, by someone claiming to work for NatWest - it is a typical lie to get the person to trust them.
I am very sorry if I have added to your worries, but as I have found out recently, there is a huge problem with this sort of thing, and since it seems so hard to prevent any of it unless the person has been deemed to lack capacity, it is necessary for relatives to be as vigilant as possible. It is possible to get a phone that will only allow certain numbers - I will try to locate a thread that mentions these.
PS - there is something called True Call nuisance call blocker - it has been highly recommended by posters here.
