There are no functions to protect the software port output enable register (SPOER) directly.
However, POE function doesn't operate only by the SPOER setting.
If the settings below are not enabled, ports are not in high-impedance state even if SPOER is set to 1.
1) Select the function of pin with pin function controller (PFC)
2) Select high-impedance for port output with port output control register (POECR1,POECR2).
Although the initial value of POE3 pins is POE, once you select the port in the initial setting, the setting remains until reset is input. The POE function is not enabled even if the POE is selected from the second time or after