You can't but I bought some inserts on eBay which you can put into the belt clip receivers to fool the system. They work. They are sold in pairs, buy 2 pairs and cut them down so they don't stick up. I'll post some details and photo's later.
The thing is, many women with heavy handbags place their bags on the passenger seat, unbeknown to them the weight of the bag operates the sensors in the seat and the sensors cannot tell if it's a person or a handbag. It thinks it's a person.
Now the same thing is happening when you carry your dog on the back seat, the sensors cannot tell if it's a person or just your dog.
The safest place for your dog if you don't have him anchored down properly is in the boot.
If you had a head on accident with the dog unrestrained, that dog could either hit the back of the front seat or go through the screen, if he's sitting in the middle.
I hope this sorts out your rear seat warning buzzer?
Caught a "dog training" program on TV the other day. Dog was getting stressed sitting in the hatch area so he moved the dog to the back seat. However he put a body harness on the dog with a strap with a seat belt buckle on the harness long enough to fit into the seat belt fastening.