I don't think it would be fraud to have everyone write down the same name. Additional tickets with your name on it do not increase your chances of getting picked, because the Goblet of Fire is not a lottery. The goblet picks the best candidate regardless of any other factors. Since no one is obligated to enter the tournament in the first place, they are not subverting any rules by putting someone else's name in (except, arguably, where the other person did not want to enter, but that's not the case under discussion here). So having everyone submit Krum's name would be no worse than having Krum as the only candidate in the first place.
However, it is also possible to use fraud to ensure that Krum gets picked. This is in fact exactly what Barty Crouch Jr. did with Harry. You simply have to submit his name under another school so that he is the only candidate and will thus automatically be picked. This would be fraud because it is inventing a new school that is not part of the tournament, and it requires confunding the goblet.
Either way, Karkaroff could have ensured Krum getting picked if he really wanted to. One way would apparently be technically legal while the other way would not, but either way Krum would have to compete once he's picked.
Thus, there was no real fraud prevention built into the goblet that couldn't be subverted. But there's still the fear of getting caught which might deter most people.