I remember in a 3200 race my senior year: within the first 200 meters i had been cut off by someone who's first stride in front of me had spiked me right in the testes with his back kick. So i layed for the first half of the race, and eventually the lead pack broke off and i was the last one to go with them around halfway through. So i start to work my way up and this kid who cut me off just wont let me through. I try going around on the straight, no dice. He drifts to the outside for whatever reason, i try squeezing in on the inside, he pushes me towards the inside rail. As he starts to come at me from the outside i go ahead of him and plant a spike on the outside of his left foot (didnt get a final look at the end of the race, but i do know that his spike was bloody with a sizeable tear). He didnt bug me the rest of the race.
Another time, i was trying to pass someone on the outside with 600 to go. The kid gave me a nasty look and starting goin wide with his elbows and caught me and it caused me to stumble. So i dropped back and dried to pass him on the straight. He swung wide so i just stood my ground and played elbows with him until he got pissed and clipped my heels. Waited for a couple of strides and then got his calf pretty bad. Finally the idea that i wasnt going to fade with 400m to go got to him.
Moral of the story, stand up to the pricks. You dont have to be a dick and instigate it, but for the love of god if they start it you finish it. It only gets worse the faster you get, and you dont have to play nice.