Seems to me there's not much point in trying to persuade the lady at the homeopath kiosk. She's a true believer. You could have bought and gulped down the entire display without incident and she still wouldn't be persuaded she's selling bottled water and sugar tablets. You might as well try to prove logically to an evangelical that his religion makes no sense.
Bump it up to the store owner? Either the store owner buys into homeopathy, also, or doesn't care if it works or not as long as it's harmless to the customers and profitable to him.
Now, if there were a dozen customers at the kiosk, and you were to buy a homeopathic headache "remedy" and start, right there, in front of everyone, silently taking one tablet after another...
But that might get you thrown out of the store, too, and there's nothing you can do about it if it isn't your store. But you knew that already.
You could set up a table outside your local TV news station offices and invite them to come film. You'd have a large pitcher of water, a couple of bottles of woo tabs, and a sign in front of the table saying, "Watch Me (Try To) Commit Suicide By Homeopathic Headache Remedy Overdose."
Be sure Eyewitless News is there and filming before you start.
Then start popping the pills, one right after another (if the bottle has a couple of hundred pills in it, you can do two or three at a time). Have snarky comments ready: "Gee, imagine what would happen if I were popping aspirins like this." "Good thing these homeopathic things don't work." "Don't bother calling 911, but could you call Gino's Pizza? I'm kinda hungry." "Gee, these homeopathic pills taste just like sugar..." "I'm running low on these homeopathic pills and I still have that headache. Here's nine bucks; can you get me some more? And keep the change."
Use the word "homeopathic" as much as possible.