I'm sure many of you know the case of Tony Stewart
It looks like he's freely continuing with an unregistered account
Can unregistered users bypass suspension, is it uncontrollable by admins?
The fact is that there is no real recourse for preventing people from registering new accounts. Or generating new unregistered accounts
This is the internet, it's painfully easy to superficially change your identity. Requiring registration fixes nothing here as you can get an open id as easy as you can get a new email address. IP bans are relatively easy to avoid as well.
SE is hard at work developing solutions to quickly identify and even preemptively block trolls from showing up continuously. But the fact is, if there is a determined human being on the other end it's going to be hard for any automated mechanism to stop them completely.