Perhaps you should be realizing by now that homework is no longer a viable way to test how well students actually know the subject matter, and the only viable way to do so is in-class tests/exercises - frequent enough and worth enough of the grade to force them to study between lessons.
OR, alternatively, you could just keep doing what you're doing, and instead, in effect grade people on:
Comparably poor grades:
* People who actually do the work by hand
* "AI N00bs" who let stuff like that slip through
Comparably good grades:
* AI-experienced people who know enough to skim over the work, do multiple passes, or issue prompts that prevent such tells from getting into the work
Wherein, in effect, your net reward/punishment is inverted vs. what you should want it to be.
Closing one's eyes and ears to the fact that the environment students operate in has changed isn't an acceptable option. Education must adapt. And watermarks, BTW, will only catch "the n00bs"; more experienced / up to date students will keep track of what watermarks and what doesn't, and use AIs that don't watermark, or de-watermarking services.