You cannot safely "disconnect" you rear brakes. There are a few ways brakes can fail... they can leak fluid resulting in no braking power, the calipers can seize up (what you are referring to as clamping onto the rotor), and they can just wear until the pad surface is gone and you get metal on metal (which is most common). Saying the brakes are "almost gone" is relative, you could have months left with very little meat on the pads. What will most likely happen is option "3", the pads will wear out and you will have to replace the pads and rotors. I would suggest that you drive the car as little as possible until you can have it fixed, the longer you go, the more expensive it will cost to fix the problem.