If you see an user that you can clearly tell has posted many questions in one day that are homework questions and they show no effort whatsoever nor do they tell you it is homework, are you supposed to flag the questions? I consider it to be completely against what the website is going for?
You can downvote, and with enough reputation vote to close. You can also leave comments pointing this out, although try not to get into very long meta-discussions on the main site.
There is a limit of six questions per 24 hours for a user, and 50 questions in a 30 days period. So any abuse comes to an end. If you do see a user opening a second user in order to circumvent this limit, this is a casus belli to flag for the attention of the moderators.
In that case, use the free form flag along with links to the user accounts and explain why do you think this user is trying to circumvent the limitations of the site.
Where multiple postings do not circumvent the rate limits, there would have to be something extraordinarily strange about them that could not be handled on a question-by-question basis by voting, flagging, commenting, answering, or ignoring individual posts, treating each post separately on its own merits.
A personal feeling that somebody else is abusing the site with multiple questions, does not automatically mean that it is an abuse, or if it is one, that it should not be handled one question at a time.
A corollary is to not bother the moderators when possible and just vote. I do not know if flags to close (choosing one of the standard reasons) create a task for moderators, or just push the question automatically into the review queue with no other implications. If it does hit the moderators, that would be one more reason to have a "request for close/delete votes" permanent meta thread, or to use the existing reopen votes request thread for that purpose.