How can I help Mom avoid scams or spending money unnecessarily on things she doesn't need without taking over her finances?

Reducing the limits on her CCs will probably only bring more problems – the CC companies allow charges over the limits and then charge enormous fees for going over your limit. Deregulation made CC companies remarkably similar to borrowing money from the mafia!
Suggest that you help her "simplify her finances" by allowing you to set up her monthly expenses to be paid online. It's less like "let me take over" and more like "here's an easier way for you to do things." Then, you can have all her bills come to your email address (don't set them up to auto-pay - that can be hard to stop nowadays). There's a good chance that, once she's not paying and worrying about her bills, she'll quickly get used to it and count on you to handle it. That's what happened with my dad even though getting him to allow it to begin with took an intervention-style meeting where my sister and I nearly had to threaten him breaking off all ties if he didn't at least try it! He was running through hundreds of dollars a month in interest and late fees and bounced check charges.
I have a friend who also had the same experience. His parents were adamant that they wouldn't give up control, but once he insisted and they tried it for a few months, they didn't even bother opening their own mail - just handed it over to him to sort through.
It seems like most of the loss of independence that seniors fear so much is far outweighed by relief from stress once we adult kids talk them into letting us help out. If you think about it, it must be a horrible feeling to know that you really can't get a handle on things each month, but feel like it's your duty (and even a measure of your worth as a person) to keep at it. Failing again and again must be so frustrating and draining of self esteem.

the scummy scan artist take many forms as well....in my dealings, every charity and donation he sent, came at least one or twice a month if not more. The mail also looks like a "bill" not a donation. Elders like surveys because this lets them feel that they are being heard. When I started to see some changes in my uncle and left his check book out, wow was I amazed!!! He was donating to the same organization like 3x a month if not more. And some of the organization were asking to be left in their wills!!! My uncle had an excess of $2000 a month going out until I took over. And it took almost a year to have the massive mailing list stopped. The marketing agencies for all non-profit and profit organization are smart. The know how to market what they need to sell. They use alot of American Flags waving as well, because anyone 60-90 yrs were all taught to be very giving and patriotic. Goes along with church affiliations as well, sorry to say. When my uncle died, some of the church members that he once attended years before, said that he donated his land to them. He didn't. Just goes to show you, when death enters, the vultures fly. Be careful with your elders hard earn cash.

In one case, I encouraged a family member to go to the bank with their parent and have checks made out that took two signatures. One with the parent, the other with the family member. No scams possible. This gave the parent the feeling of some control and yet no excesses were spent. SK

Dear cl,
The problem you're facing is a difficult one since your mother has been independently living all these years. Since she voiced her concern, it sounds as though she may be receptive to your help. Rather than "insist", possibly she will verbally agree to allow you to have on-line access to her bank account? (Of course, something in writing would need to be secured, but it sounds as though a Power of Attorney appointment might be in order if you are noticing cognitive changes.) Mainly, she needs to know you will help her and have her best interest in mind. Most elders know people who have been 'set aside' by their children, and misperceptions sometimes become the barrrier to help that is needed. The fact is that elderly people are vulnerable, and targets for the scummy scammers that are always out there. Maybe you could speak with one of her friends and get your Mother to agree to telling that friend before she writes any checks or makes any purchases over.......$300 or whatever you think reasonable. Mainly, your mother needs to know that assigning a Power of Attorney is not relinquishing her ability to live independently. Most doctors offices have paperwork for a living will; maybe she'll agree to ask about it. Good Luck.