How to prevent hacking a circuit?
This answer will be ish on point. But at the same time not. It's half on topic and half off topic. Sadly it will also be somewhat mainly opinionated. This is more of an attempt to persuade you into doing the exact opposite. 



This is one of the reasons why I decided to focus on electronics rather than programming, because "no one can steal an electronic circuit". 

I totally forgot about schematic designs and embedded software. You can even decap IC's if you want to have a closer look. Add an electron microscope and you can see, in real time, what a decapped IC is doing.

With that said, everything can be stolen or hacked one way or another. We're humans, we're smart. We never give up.

Around the same time I came to terms with that truth I also saw this youtube video.



When you finally understand that if you make good things, people will "steal" it, or more correctly copy it. It's not necessarily a bad thing. You will get free PR, more people will hear about your product, more people will want to support you and buy your things.

Just look at Arduino's website, even they are selling Teensy's which is almost a clone of Arduino's. 

This is why I hate patents so much, because it's hindering innovation. Just imagine where we would be today if there never was a patent for 3D-printing. We've lost about 50 years of 3D-printing development because of some patents. That's dumb. 



Let's say you come up with some futuristic design, let's say you can make ultrasound powered by just 0.7 V. In other words you can remove bacteria from water by powering it with just a solar cell. People would go mad about that design in 1 st world countries. Especially hikers and what not. You would make a fortune.

Your product would be used as a luxury product, because instead of bringing water you can just take some water from a disgusting lake and drink it knowing there's nothing nasty in it. Your product has purified it. 

Then someone from china buys your product. Reverse engineer it and find the schematic. This person mass produce it and sell it on aliexpress. Suddenly people in Africa can afford to buy your overpriced product. Now, are you really mad? A product that you have made has made the life of millions in 3rd world better. Instead of having to walk 6 hours to the nearest reasonably low-bacteria water well. They can take water from a lake 30 minutes away with bacteria in it. 

In this case it's not a luxury product, it's a product that changes lives for the better. At this point you will have millions of $. But so has the person from China who copied your design. Had that person sold at your price, that person would have had maybe 1 billion, so you've "lost" 999 millions. Do you care? Should you care? I wouldn't care. 



So no, don't hide your designs, don't make it secret.


Don't scrape off the serial numbers off your parts
Don't Taint the components with something, not even black ink.
Don't try to corrode circuitry when exposed to air.


Share everything on github, hackaday, instructables, whatever, allow random people from anywhere over the world to pitch in / help. Make it open source. Don't patent anything. Make the lives better for everyone in the entire world. 

Did Volvo patent the seat belt? Or did they allow their competitors to also use it? 

Is all the Linux distros free and open source?



Or do you want it to be like it is on youtube right now? Some silly algorithm that no one understands how it works that is making wrong left and right and demonetizing videos, making youtubers lose money who has done nothing wrong.