How to repair / create a CF seat post clamp interface?
It's a bad idea to sand down or modify the carbon fiber of the seat tube. You don't know what you will do to its integrity or strength.

I would do some research to find out if a replacement aluminum seat 'cap' that accommodates the clamp can be bought, and the proper technique and epoxy type for bonding it in.

If you choose not to fix the frame, don't junk it. Stick it on Ebay with the appropriate disclaimer about the seat clamp. Maybe someone with the required know0how will but it up and fix it.