I would say your position as Head Cashier has no real meaning. I would sit down with my boss privately and lay out your concerns. Let him/her know that you need their back up and are having a problem with certain cashiers. Let them know that if you are the head cashier, you need some kind of power and back up from them.
If you are very polite and lay it all out, they may respond well. But without the backup of management, you are pretty much out of luck in getting the cashiers to cooperate.