If you really hate these messages showing up in your logs, you can configure a firewall rule to block the SSDP packets which cause them. Something like the following should work on most distributions:
sudo iptables -I INPUT 1 -p udp --sport 1900 -s 192.168.1.1 -j DROP
This should cause packets from 192.168.1.1:1900 UDP to be dropped before reaching Plex Media Server. The rules you add are not persistent by default on most (all?) distributions, but there are ways to make them so.
Alternatively, you could check your router to see if UPnP is enabled. If it is, and you’re not using it, disable it. This should prevent it from responding to PMS’ queries.
