There is no way to completely disable Plex’s use of SSDP discovery. It is used for other functions in addition to client discovery (which actually can be disabled). The best solution is likely for you to block the responses from these clients, or to UDP 1900 on your server, using iptables or UFW or whatever you use as a firewall. I give an example in this older post (it’s old, but was tested to work at the time).
