I think you'd have to elaborate on the problem. This sounds like something you should be doing differently, but I don't have a Nest camera. The title itself was enough to shock me to try and find out what's going on for my own awareness.
But if Nest cameras work like any other Google/Nest hardware, then you have to be on your own wifi network and Google Home account to make any changes to your hardware at all. If the hardware has no internet, then it typically creates its own wifi connection, to which you can assign it to a new Home/account, but I'd imagine you need physical access to the hardware to confirm a number or something similar.
So... either you have uninvited guests on your network (change the password and maybe also the encryption method), a member on your Google Family/household or user in your home is doing this, or the hardware is losing internet and someone is taking advantage of it.
