...on a side note, if some guy spins underneath you to block you from taking off outside, is it in your right to just run them over?
Slant your take off and drop in over their shoulder if you can. A lot of people seem to think that they can spin and get in front of you when you are already paddling and committed. I try my hardest to keep this from happening via an the "angled right-over-their shoulder" drop in; they usually eat it. I've had couple people get mad at me for doing this, but I remind them of their behavior and they usually shut up and paddle away.
That said, I have been burned and it will happen again. Just too crowded sometimes. Last Saturday, a guy dropped in on me and when I said "hey", he fell and shot his board right at me. I didn't need to say a thing to him though because his buddies ripped him a new one for it. I just laughed and went back to my line-up spot.