death to capslock

Exactly. It's also a very handy way for Severus to harmlessly blow off steam. If Harry ever stopped to think for a moment, it would be clear to him that Severus (obviously) doesn't have the power to unilaterally expel him. If he paused for another second, he'd notice that there's never any hint of Severus even attempting to carry through on his threats and perhaps (assuming he thinks about it at all) conclude that they are, in fact empty and not to be taken seriously. Indeed, with even a slightly more reflective and less CAPSLOCKY student, the repeated making of empty threats would be a quick way for Severus to *lose* any sense of respect they might have had for him. I think Severus does it because he knows *Harry* will buy it as a credible threat (and thus hopefully start *obeying* him), and because it's a release valve - he'll take them where he can find them. He's not actually obsessed with expelling Harry, as his conduct with Fudge in POA illustrates: it's his one major opportunity to go over Dumbles' head and really punish the boy, and he immediately says Harry and co. aren't responsible for their actions. He's letting off steam in a safe (and probably somewhat amusing to him) way while still attempting to make Harry recognize him as an authority figure. If Fudge or Dumbledore or Umbridge, etc. ever actually tried to expel Harry, he'd never allow it.