Having a markdown file is just as effective as the robot.txt files were for webcrawlers. It assumes the rules will be followed.

Poisoning is better, not only because it infects the very thing they're taking, but it's not a defined thing the model can be instructed to watch out for. Since models do "think", they could be told to look for such attempts and avoid it, but they still have to have a template to understand WHAT to look for.

Now the question is, how to best plant such things and make sure they look very much like code to be taken in. Comments are easy to avoid, it needs to be part of the code while not affecting the code working.