StackOverflow probably needs a function to flag users, not only questions and answers.
Case in point: SO tools: new posts by recent users.
Looking at the few first pages, it appears that almost all new users are spam bots. It might be possible to counter this wave quite quickly but the general problem persists and marking their messages as spam rather than the user is a very indirect way to fight this massive amount of spam.
I don’t know whether the answer given to question 1323 applies here, since the situation is different: until now, the question was how to deal with disruptive people, which are relatively few in number. Against bots, this clearly isn’t enough.