This happened to my Father for several years before he agreed to go into a Retirement Place.
Even though he had ran his own business for 50 years, was very prudent and managed bills, shopping etc.he was still conned by these people coming to the door, offering to trim trees, clear his gutters, fix his roof.
I live literally a minute away and used to plead with him to ring me when they came to the door but he just wouldn't, although he always told me after, insisting they'd done a good job.
He also started giving his card details out over the phone when call centres rang, selling insurance or whatever.
We solved this by scratching the three numbers off, on the back of his card, so it couldn't be used that way but still allow him to get cash out or pay for things in the usual way.
If I had thought at the time, I should have had cameras fitted to cover his doors, the ones that would alert me on my phone if activated, so I would have been able to fend off these thieves.
It could have possibly put them off as well, thinking that they were being recorded.