1st ask to deactivate. Give them some reason which should be/sounds genuine. And ask to return back balance. They'll call you back in 2-3 days to confirm name, address for cheque and reason for double check. Once this is done it'll take 3-4 days to get cheque. Local dealer guys will bring that to you at your home. If they insists on taking set top box then give them. Else tell that customer care told not to give STB or anything. Been there, done that. It was flawless procedure.
By the way if you are not using STB you can parcel it to me as my son requires kids pack.