This is a great question -- I always want to do things like this, but avoid doing so in order to limit my own exposure to legal/ethical second-guessing

Removing your cheater from the class would be kind of a dick move; all their little classmates are probably cheating too. Making the student repeat the assignment and giving a finger-wagging lecture about cheating to the whole class would probably be fine. Lotta effort to go through to catch one of the people cheating in class though.

By flagged, do you mean incorrect, or along the lines of embedding something into a PDF?

@cag51 correct solution but with some additional embedded data that identifies the source like metadata, comments, and in the case of code extraneous steps that don't break the solution, but don't add any value.

How do you know the actual identity of the person who posted the query to the web? Mr. Jones could easily have used Mr. Smith's name on the web site.

It's not rocket science. If you give an answer to someone on the internet where one of the variables is named zyjkbk and someone turns in an exact copy of your code down to the variable name zyjkbk you've found your winner.

A clear warning is necessary - depending on your relation to your students you could: Place a bid and hand in a flagged solution, buy a lot of cake and coffee from the moey you gained, have a party with your class and tell them how you got the money for that. The student should just go away anonymously with zero points (but no further consequences). This story will be spread for years!

@OBu That seems like a really fun idea and one which could easily lead to administration being very nhappy.

@JoshuaZ I would say at our university that would work out - but I agree, some people might go crazy at other universities. This is why I did not post it as answer...

What happens when the marked solution appears more than once? Students tend to communicate, and you couldn't possibly fail all of them.

What happens when multiple people cheat and are caught? What do you think happens.

@CJ59 If the only penalty for cheating is that they have to repeat the assignment...which is to say, they have to do the assignment this time, then the only difference between them and the students who did their own work the first time is that cheater is able to turn in the assignment after the deadline. That won't discourage anyone from cheating. But if they fail the class because they got caught, that might make any other cheaters who didn't get caught think twice before doing it again.

corresponding question for the legal side: law.stackexchange.com/q/36748

Why are there 2 users called Ray with different ID's and user pages answering queries about this question???

@Ray This isn't exactly the place for this discussion. Practically, as an adjunct, if you make a bunch of work for people because you file a bunch of formal punishments for plagarists or cheaters, this will be your last semester there. You basically have to handle it in house, which limits your options.

@CJ59 If a student hires a freelancer to write a program for them, and the freelancer names one of the variables zyjkbk, and the student doesn't ask the freelancer "Why in the world would you name a variable zyjkbk?", they deserve to fail.

I came across this exact situation while TA-ing; the professor had reused homework from last year, but the answer key contained basic arithmetic mistakes that "somehow" were exactly replicated on a student's homework. We realized that this student was already failing the course (F) so we decided to just give them that grade. Not sure if it was the most ethical course of action but it wasn't terrible and it was easy.

When I was TA'ing a number of years ago, we discovered that a number of students were outsourcing their (Java) homework to China, but we didn't know exactly who because we only had usernames. The professor in this class happened to be a Chinese national and so he took the bids and gave them all the same code, complete with comments in Mandarin. We got about a dozen (out of ~200 students) copies of that code back - some of them hadn't even bothered to remove or rewrite the comments. Easiest way to catch cheaters ever. Note the prof. did not actually take their money - that would be iffy...

@Murphy Displayed usernames aren't unique (or immutable). The two of us aren't even the only Rays here, and our numbers continue to grow. One day, everyone on this site will be a Ray. Resistance is futile.

If you're feeling mean sell em some wrong answers. When we shared material at uni to we took pains to write our own comments and variable names, especially when the level of sharing exceeded what might be regarded as collaboration. Use a pattern of compensating errors to watermark the material as this is more likely to go undetected -- and if they fix the bugs this demonstrates the understanding for which you are testing.

"one of students had posted their assignment on a freelancer website asking for the solution." This might not be unethical. It is possible that the student has already cpmpleted her/his assignment and now only wants to see how professionals would do it. No, I don't even buy this myself, but you have to consider this view as well.

What's unethical is the student doing the cheating. People like this squeeze through college, then go into the workforce and make other people who didn't cheat look bad. Cheating devalues the credentials of your institution, and undermines everything you're trying to accomplish.

@ScubaSteve: I guess it is consensus here that a student who cheats is acting unethically. But Person S doing something unethical or even illegal does not make person T's unethical or illegal conduct any better. So knowing that the student does something that is unethical does not help answering T (OP's) question whether they are acting ethically.

@ScubaSteve: Maybe there were some great cheaters around which I did not detect, but both from having been a student at some point and then later the TA perspectice, but the ones who cheated (or wanted to or tried to) did not impress by any good marks they got. Quite the contrary. But I have to admit that over here homework is typically considered an excercise offered for the benefit of the students and rarely enters the marks heavily - so from the side of the university noone really cared whether a student did their homework or not.

@cbeleites - Marks are a bit of a red herring. Employers don't generally look at marks, so if you hire someone from an institution who cheated their way to their credential with bare minimum marks, and they turn out to be incompetent, then maybe you think twice about hiring the next person from that institution.