Just log into your routers admin panel and see if there are any online devices online that arent yours? That would be my goto because then you can just block them from the panel?

otherwise via linux cli you can do arp-scan and youll see online devices in your subnet. Theres also a few options to scan different interfaces

Wow Now I really want to buy siding for my house!

Is there free shipping from Fairfax to South Eastern Europe? Otherwise why tf would you try to advertise to me?