If you put code online publicly-readable then there is a signicant probability that it is allowed to train ML on it. So the first step you need to take is not d

If you put code online publicly-readable then there is a signicant probability that it is allowed to train ML on it. So the first step you need to take is not doing that. You must ensure that anyone that reads your code has first signed an agreement not to train ML on your code. If like GPL you want to allow people to share your code further, and make derivative works, then you must ensure that those people lock such works behind restrictions in the same way, such that derivative works can also only be read after signing the license.