Need to end auto renewal of several magazines, any ideas how to stop them?

Automatic Renewal will never stop unless told to do so and as long as they're being paid for.

You have to call each one to cancel.

If after making the cancelation call, they keep sending them, then
Close the source where the magazines are getting pd from.

If it's on a Credit Card just conveniently lose the card and once they reissue a new card, no automatic payments will be on it at all.

Yes......contact the magazines directly (Google the name of the mag and 'customer service phone') and tell them you want the subscriptions stopped immediately. If you can figure out which credit card or better yet, debit card MIL is using for the renewals, simply call the bank and tell them you don't authorize further withdrawals for the magazines. Get HER on the phone if necessary. Who is her financial POA? That person should be able to make all the necessary changes to these subscriptions.

Who handles her accounts? They need to watch the charges, and when the charge comes in, call the card company and get it removed.

SorrynotSorry, first check the mailing labels on the front of the magazines to see exactly when the subscription expires.     It may be that they're for several years, in which case you would have to cancel them specifically.

Some of these backhanded outfits, as LeaLonnie wrote, do use tactics to kind of trick someone into agreeing to automatic renewals.

Call the credit card/bank company, explain and get charges reversed. A lot easier and faster than trying to get the magazines to stop. Ask also how far back they can credit you--might as well ask just in case...
See if you can get yourself added to the card or account as a non-charging authorized user. Think that is the wording. That lets you take care of these kinds of issues, get the bill emailed to you if you wish, yet won't raise issues that you were using the card.
I also made changes to the cable subscription for my mother and saved nearly $2K/year.
I'm sure there will be a reward in heaven for all the work we are doing for our parents. It's not likely to come here on earth!

Call each magazine directly so you can track back to see where the charge is coming from and turn it off.
Until then, maybe a local nursing home or other facility or agency would enjoy receiving magazines? Someone might even pick them up.

You can contest each of the charges for each magazine and that will probably put an end to it but it requires some work. Or report fraud activity or her credit cards as stolen so that those accounts are closed immediately. Does she even need a credit card? If not I'd ditch it completely. One less thing to have to worry about.

Closing/cancelling her credit card is the fastest, easiest thing to do. Don't make more work for yourself than necessary.

Inside the magazine, or if you just google the name of the magazine and customer service you should get some contact info. You will need info on the mailing label or insert for them to identify the subscription and tell you the source of payment and what's set up. If these renewals are happening via a credit card in her name, you can also notify the credit card company to not allow any transactions from the source. You may have to say you are she:-) or provide documentation of being POA which will get more complicated.

Some libraries have a box for "take what you want" and you could leave those unwanted issues there for someone to enjoy...

Calling separate magazines may not work. My sister, years ago, got stuck with this problem. Its a scam. They call and tell you that they can give u a good price on 5 subcriptions at, lets say $20 a month. Sounds good but you are actually paying more than an individual subscription. 240 ÷ by 5 = 48 a year. At the time you could get a subscription for $20 a year. My sister had to call the scammer because that who was receiving the money. They told her it was a five year obligation. If she wanted to cancel she had to pay the balance due.

If this is being withdrawn from a bank account, ask the bank if the withdrawal can be stopped. If credit card, freeze it so no more charges can be made. Make credit card aware that the card is not to be used.

Hope someone has POA to be able to do this.