To clear the AES key manually, disconnect the Vbatt pins and power-cycle the board.
Note: Pressing or pulsing the PROG pin when the board/FPGA is powered up will not clear the BBR register.
Alternatively, you can clear the AES key in Vivado IDE by right-clicking the FPGA device in the Hardware window, selecting Clear BBR Key
Figure 1. Clearing the AES Key for UltraScale, and UltraScale+ Devices
When the Clear BBR Key dialog box appears, click OK to clear the key from the device
Figure 2. Clear BBR Key Dialog Box