If Ubuntu gives popup messages about auto-adding “new” printers upon connecting to a wired or WiFi network, try the configuration below.
Stop auto-add printer on network connect by editing
Then restart CUPS from Terminal:
service cups restart
Test: 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.