This is not the case. When you close vents and registers, you block cooling and heating from getting to that room. However, your HVAC system still has to work to cool or heat that space, using the same—or more—energy as before.
There are additional impacts of closing vents, as well. For one, your system needs proper air intake to function: blocking off air intake channels can lead to issues such as frozen coils in the summer. Second, the comfort of your home could be negatively impacted, as an imbalance of hot and cold zones in different rooms is created by the blocked vents.