Nick, is this a local group or a national group?
If they're national, notify their national HQ immediately. No national condones this kind of behavior.
If they are local, get together all the fraternities and sororities and place an ad in the campus paper (signed by all of you) that says "The Greek community of XYZ U. does not recognize or condone the actions of the group known as Rho Rho Rho. They are not a part of the XYZ U Greek community."
That way hopefully people will realize that this is not the way the rest of the Greek orgs operate. And if the bouncers at the bar want to throw them out, hold the door open.
