Securely transmit commands between PIC microcontrollers using nRF24L01 module

Securely transmit commands between PIC microcontrollers using nRF24L01 module

I have created a small wireless network using a few PIC microcontrollers
and nRF24L01 wireless RF modules. One of the PICs is PIC18F46K22 and it is
used as the main controller which sends commands to all other PICs. All
other (slave) microcontrollers are PIC16F1454, there are 5 of them so far.
These slave controllers are attached to various devices (mostly lights).
The main microcontroller is used to transmit commands to those devices,
such as turn lights on or off. These devices also report the status of the
attached devices back to the main controller witch then displays it on an
LCD screen. This whole setup is working perfectly fine.
The problem is that anybody who has these cheap nRF24L01 modules could
simply listen to the commands which are being sent by the main controller
and then repeat them to control the devices.
Encrypting the commands wouldn't be helpful as these are simple
instructions and if encrypted they will always look the same, and one does
not need to decrypt it to be able to retransmit the message.
So how would I implement a level of security in this system?