Sure
first: there are many people which don't understand the concept of dlc and enter ga expecting the game.
second: people don't read the abstract/description of the ga. (yep, I rarely do it myself)
third: (still a major weakness of IG ga and see second) region locks are not mediated in ga
fourth: it is still easier to do a mistake in making a ga than to fix the outcome (which is close to impossible or really expensive time wise)
fifth: there are people blaming you for making a ga on false premise (bad luck if you make a ga for a bundle leftover if that game is/has been given away for free)
sixth: there was a phase in the IG giveaways where people did legit ga but received a negative feedback. The assumption a that point was that key farmer tried to kick out other key farmer reaching higher levels, thus reducing the competion.
The feedback button is/has been often misused/abused. The collateral damage is still felt today (well ... there are other reasons too)
Things are getting better (at least my feeling), would be nice though to see the actual feedback of a ga.
Blacklisting makes only sense if any of the above is defused, otherwise it is just a extention to the feedback button.
Edit: wording and formatting