From my experience working at a university (in the foreign language department), I found a number of students who put a large effort into "getting out of work". I was astonished at the number who put more effort into avoiding work than it would have taken to get an A.

If I were in this professor's case, I'd just mark the answers wrong, and in the future upload more wrong answers. The students who use these sorts of online dumps aren't the ones who study and will beg at the end of the semester for some sort of extra-credit. If it were necessary, perhaps have a second gradebook where the number of "exactly the same wrong answer as the bait" were kept.