I have had success with:
“What are you using to treat the cracks? What causes them? Just in case it ever happens to my horse. And are you seeing an improvement? Oh, no? Well, my farrier will be here on Monday, should I give him your number - he’s a nice guy, he’s likely to fit your horse in even though he’s busy. Cool, he’ll give you a call”
I just pretended I wanted to learn from them and that put them in a positive frame of mind and they were receptive. From then on, I would have my farrier phone them every time they came out to see my horse and they were pretty much shamed/organized into behaving.
You should adjust the approach based on their personality.
Don’t pay for them.