Is there a way to turn off LibreOfficeUpdate via terminal?
You could try blocking all outgoing traffic of the application with the firewall (ipfw) or you can edit the hosts file in /etc/hosts to route the update server to localhost (127.0.0.1)

I don't know the specific update servers, but that would be not that hard to find out.

(for exampple here : update.services.openoffice.org -- so there should be a possible way. Be careful though because libreoffice/openOffice sometimes produce the error "The host name ... could not be resolved.")

Try also experimenting with the GUI on your Mac and the logfiles or list of open files (lsof)