Ubuntu 17.10 gives popup messages about auto-adding “new” printers each time I connected to a wired or WiFi network. I don’t have this problem on Ubuntu 18.04, even without the changes below (factory defaults are OK in 18.04).
Stop the auto-add printer on network connect by:
restart CUPS from Terminal:
service cups restart
Reboot PC with network disconnected, check that the offending printers are not in Settings → Printers, and then connect to the network and verify that the printers don’t auto-add again.