How to block off and disconnect/remove the rear A/C unit

It depends on which kit you have and if you are removing both the heat and A/C parts. You also may want to keep the rear A/C as most vans get pretty warm inside, even insulated well. The A/C then can work with the front system to cool it down.
If you wish to remove it you will need to determine if your kit goes on the compressor or the rear lines. I have seen both. Then you need to recover the refrigerant from the system, disconnect the lines and cap them off. If it also has the rear heater you will need to drain the cooling system as well and then you can disconnect all of those parts.
Once you have them out, you cap the unused ports and recharge the A/C and refill the coolant.