Yes, it would already help if they started with whitespace and punctuation. That would already give a big clue as to what is AI generated.

In fact, using a different scheme, we can start now:

    U+200B — ZERO WIDTH SPACE

Require that any space in AI output is followed by this zero-width character. If this is not acceptable then maybe apply a similar rule to the period character (so the number of "odd" characters is reduced to one per sentence).