IRS regulations regarding Pre-tax Health Insurance?

I have decided to drop one of AFLAC supplemental health insurance policies offered by my employer. Love AFLAC, but need to increase my takehome pay as much as I can. Times are very tough. My HR person was about to fax in the cancellation, but called the rep first. The rep told her that I cannot drop it until open enrollment in January, because the IRS says so. It's a pre-tax policy. This seems odd to me. I can't get on the IRS website for some reason. Does anyone know if this is true, and why the IRS would care?