That's sad. Wattpad prevents copying contents atleast on their app so I wondered if it could also be applicable here. If you do not mind explaining a bit more so I could understand. I have no idea how programming works.
Basically, there are multiple ways you can try copying a chapter.
The simplest method possible is to have a bot that copy pastes the content of the chapter from the webpage and posts somewhere... Those can in theory be stopped by stuff like copying protection.
However, it's ridiculously easy to bypass those copying protection thingies. Not to mention you can just press F12 and get the text directly from the page's code... So you need a more complicated protection method if you wanna stop bots.
But even if you do make a complex protection method that can actually stop things that try copy pasting what's written on your page... You can never stop a program that translates what is visible on the screen to text. Which is an OCR program.
Basically, an OCR program will look at the entire screen as if it was an image, and then it will identify the letters that are within said image and transform it back into text. It's basically impossible to stop a program like that as this would mean making the text illegible for your normal readers as well.
In summary, it's impossible to stop someone from copying your chapter if they want to use a good program to look at it. So the only thing you can do is to ignore the existence of thieves and focus on your own writing instead.