When I created my Steem account on 2017.05.17, the Steemit website asked me for my SMS number (and as far as I remember, it sent me a verification code), so I (and many other people) was not anonymus at the time of the account registration.
But I understand your point.
I think that this one (the abuse around the rewards) will remain one of the disadvantages of decentralisation.
Or at least I currently do not see any proper solution for it.