I see you’re on a Linux box… I was seeing this spam my Plex logs recently as well too and starting digging around. I saw this thread, decided if Plex wasn’t going to fix it – I’d stop it myself…
So I went as far as to block outbound UDP on port 1900 because as far as I am aware I am not using any SSDP-related features of Plex and it kept hitting various Ubiquiti devices on my network for no good reason.
sudo iptables -A OUTPUT -p udp --dport 1900 -j DROP
I mean, no more SSDP spamming but also now your logs get a whole new level of spam 
WARN - NetworkServiceBrowser: Error sending out discover packet from 192.168.30.138 to 239.255.255.250: Operation not permitted
WARN - NetworkServiceBrowser: Error sending out discover packet from 172.19.0.1 to 239.255.255.250: Operation not permitted
