Sweetie,
You let yourself be victimized by people you barely even know for absolutely no profit.
I mean I don't mind getting screwed from time to time, but ask me first, and gimme a little foreplay to put me in the mood. These girls gave it to you raw and then complained that you weren't enjoying it enough to want to do extras for them.
When ever I let myself get victimized because I was either timid or wasn't prepared for the situation, and we have all been there, I do some soul searching to find out why I allowed it to happen. And then I mentally rehearse a more appropriate response so not only that situtuation can be handled but others like it.
I have long since stoped feeling any obligation to give rides. Although there are many times when I'll pay their cab fare home from a place when I don't want them in my car.
One of my favorite responses is: "which part of NO didn't you understand?" or just refusing to move, or unlock a door, or further discuss the matter. If I have decided not to do it nuclear weapons are not going to compell me, let alone most people's feeble attempts at guilt or bitchy intimidation.
For future reference: ZTAngel was absolutely right, once they start calling you names like that either just stop and get them to behave, stopping has a great effect, or kick them off (police help may have been a bit much if they were drinking, wierd repercussions if a sorority were suspended because you were pissed when you didn't have to let them on anyway.) but they would have gotten off eventually if you just stopped and started napping.
As far as an official complaint: you still violated procedure, and anyone with a clue, and that isn't your friend, is going to wonder why you let them on and why you continued to deal with them(at least guys would).
So I would try a different punishment approach. I would use guilt. Most people are responsive to it. Personally, I have dated too much to have guilt gland still, but thats just me. I would have your sorority draft an official looking letter than states that you as a the driver bent your own rules to make sure that the girls of XYZ got safely to and from the party because you respected and admired them a lot as people, Sisters, and an organization. And then I would describe at lenth the way you were treated naming names where possible, and further emphasizing that despite outrageous provocation you still hand delivered them to their doors like the weak-willed, snivelling, incompetent, sluttly, litle-daddy's girls ,who probbaly don't even know how to open their car hoods without AAA help, that they turned out to be.
Well on second thought I would leave that little part out

. But you get the drift.
Before the letter went out, delivered in person and privately to their president, and in the next 24 hours, I would get the Fraternities to agree to mix with you and another group unless XYZ provides ALL the shuttles next year with independant drivers for your girls. They will probably go for it if you send someone that has sufficient personal attachment with them and you threaten to pull out yourselves.
That way you don't deprive their new and innocent girls of the experience but still make sure that the bad girls are held accountable.
Hope things work out for you.