You Click on the node. A picture pops up with say 4x4 squares. In one of those squares is a picture of the material you are gathering.
You click that picture and get the item. A grid pops after. Then a third. Then the node is spent.
There are prob a million ways around that too. But just a thought.
IMO just ban bots faster, don’t do it in waves months later. Ban them the second you investigate them and find out they are botting. Of course, they won’t, because if botting is profitable, people will pay for bot subscriptions and new xpacs to bot the new materials for. If they make botting unprofitable, WoW also becomes less profitable (or does it? Would enough people come back if there were no botting to make up the difference? I vote probably not).
…why not gathering tools that degrades depending on rareity affecting their durability? 
“Which one of these doesn’t belong?”
“Motorcycle, Bicycle, Moped, Elephant, Unicycle.”
Incorrect, you may not mine this vein, try again.
I have a few better ideas;
Extreme: make all herbs and ore account bound. You can give them to your alts for yr use but not sell or trade them.
Less extreme: put an increasing time limit on gathers. Each gather incrementally adds more time between being able to gather until it reaches a 24 hour period until you can gather again. Probably per character because it will still throttle bots due to their automated nature.
Even less extreme but annoying: include some form of interrupt to each gather. Perhaps a small npc spawns and attacks the gatherer. It would be an easy kill but the nature of botting software is affected by being attacked.
I have other variants to these, some fairly weird, but I think the capcha thing would be too frustrating for normal players.
Please no.
I really don’t like Captcha codes. I can live with occasionally using the authenticator but my last MMOs had captchas and it was super annoying. Didn’t even do the job
a really bad one
There are some terrible ideas in this thread.
I have a better idea; lets make bots part of the game. 
Then you won’t feel like you’re at a disadvantage and you get to have Vulpera following you around helping you out. 
Just unplug the servers that will stop them
lol that wouldn’t stop them.
Implement a system that forces you to scan your thumbprint for every node, herb, or skinning corpse.
Captcha doesn’t work because of pictures. It analyzes your mouse movement. Four squares probably wouldn’t be enough randomness to detect.
CAPTCHAs are a problem for some actual people, and there are AIs who have successfully passed CAPTCHAs in recent months.
All they really need to do is make the game check if someone has done more than gather and mindlessly attack people in the past hour (not tell players that’s what they check), provided the person isn’t afk, and if they haven’t, initiate a logout.
A real player would cancel it, but a bot would not know they have to.
If they logout, flag the account for a more thorough investigation.
I feel like we’ve had this thread repeatedly of late.
AI is already making Captcha’s useless. So it wouldn’t work.
I shall answer this in as many languages as I can
I don’t know many languages.
That would be a horrible idea. It would make people miss bots. Sometimes going overboard to stop a bad thing results in far more annoyance than the bad thing was giving in the first place.
Blizz fully endorses and condones botting. Why would you put in a captcha to annoy regular players?