The PxeServerMonitor thread is controlled by the same pieces that run the rest of the PXE server logging unfortunately. Because of this I can't get you a specific config that will explicitly turn it off... Your only real option is to adjust the default PXE logging level in the <install_dir>br/deployments/_pxe/log4j.properties file to a different level which would omit these messages. This change, however, would also hide most of the useful information you'd want to see like the PXE server responding to boot requests for example.
As a workaround, why not implement something like this via NSH:
tail -f pxe.log | grep -v PxeServerMonitor
Which would leverage the grep utility to hide those lines...
Will log4j take logging to a pipe?
I'd figured on using the pipe for the command window. I was kind of hoping there would be some easy setting somewhere that would keep it out of the log for good. I appreciate the insight.