I find in the expert online races punters only account for maybe 5% of the racers.
You go to beginner or intermediate, its well over 50% that race dirty.
Stick to the expert races and you should have very few problems. And if you lose, its not like you need the prize money, its so easy to come by in GT5P.
About people ramming you in before t1... well i guess thats usually because people misjudge the braking zones. In the past; if i saw the car infront brake late, regardless of what car i was in i would have followed suit - causing me to either hit the other driver or lose control and go off myself, usually the latter. Now i am a little more careful, if there is a car close infront, i will brake earlier than my usual zone. This often means the car infront steams ahead through the corner. But if its suzuka, it gives me room to take turns 2-6 with my usual flamboyance without worrying about shunting the car infront.
In short, there is no way to stop it. But avoid doing it to other people and maybe some guys will return the favour. Set and example and drive cleanly.