Ok, here goes... [devil's advocate]
Was it a double white line?
If not, then it's not
illegal to cross the white line in the middle of the road. Yes, it gives bikes a bad name, and the person crossing it is in the wrong if a collision takes place, and they're responsible for making sure that the road is clear for the overtaking move.
[/devil's advocate]
Before the race in the briefing the riders were explicitly told that crossing the white line on the main road out to the circuit would not be tolerated.
However, if the race rules state "Thou shalt not...", then they've signed up to that, and they must be made to recognise that they've transgressed.
Bravo for enforcing the rules.
One day, all racers will have to carry Galileo satnavs (accurate enough to show which side of the white line you are), and post-race scrutiny can disqualify them. If they want to carry bike-cams to prove their innocence, then feel free.