Its not clear ( to me at least ) what you are asking , if it’s the case the property where you live , and are leaving for a few weeks has FTTP and will be empty, but you want to stop anyone entering that empty property and ‘using’ the broadband while you are not there , why not unplug the router and lock it away ( or take it with you ), or remove the power supply from the ONT and lock it away.
If a house is empty, and has a broadband connection ( it’s pretty irrelevant that it’s FTTP ) if no one has access to the house then obviously they will be unable to connect via Ethernet , and anyone outside the house cannot access it via WiFi , presumably it’s would be a secured network and they wouldn’t know the WiFi password , then there would be no WiFi usage or broadband usage , it’s possible the ‘broadband’ may have some housekeeping type data usage, and obviously there is a tiny possibility that someone could attempt to hack into the WiFi , but that’s a pretty remote possibility, and totally removed if there isn’t a router transmitting WiFi
