It is impossible in a decentralized system to limit account creation, except as it is currently limited by time, and/or funding requirements on Steemit.
At least I can think of no mechanism that might do so. I am not the sharpest tool in the shed, however, and suspect brighter eyes than mine carefully looking at the issue might well find better ways.
That's fresh thinking on the matter, and exactly what I meant when I said someone else besides me might have ideas I couldn't come up with. I am aware that, in principle, the economic constraints on Steemit that cause new accounts to need to be delegated Steem in order to make posting possible, are mechanisms that might potentiate such limits as you discuss.
I'm not sure of details, but that may be the right way to handle spam and plagiarism. At least it's an avenue that has potential.