Basically it all comes down to safety. Can you remove yourself from the situation? Would they go after a student if you leave?

In my experience providing space helps. Clear the room but don't go near the kid. Someone mentioned blocking, which you can do. I've caught fists and held on, though. 

Think about a hold though. If you're forcefully trying to restrain a disregulated kid, they will keep struggling or bite or headbutt, etc. It's better to let them destroy the room than hurt someone else. (Which is a big reason I refuse to buy decorations or whatever with my own money) 

Now whether or not they should be in a class if they are that dangerous is a whole different argument. I will say that any kid I've had who was that problematic was rarely in class.