In newer PSU's the -12V line is no longer used, but in some versions of IPMI it's still being monitored. In this case you can safely remove the sensor from IPMI using the IPMICFG tool.
1. run IPMICFG –SDR to get list of sensors and the sensor #
2. run IPMICFG.exe –SDR del 1 (to disable sensor #1 for example)