It's too inconvenient to downvote everything. At some point, it just feels like not being worth the trouble, you've already done it too many times, besides you won't even read what the user writes. A community analogue of banning a user must be a judgment about the user, not a judgment about specific comments. The number of users who disapprove of the user, not of specific comments. This requires a new feature to do well. Could be as simple as vote up/down buttons on the user page.