Yep, banning would have been an alternative solution. But with the choice they made, it requires more human intervention, both on the shorter term (i.e. policing comments to find trolls and mark their accounts) and the longer term (i.e. policing marked accounts and unmarking them when their comments become consistently high quality), and that's a lot of work, and too subject to flawed bias, which is why it's a flawed choice.