Unfortunately, the new smartthings integration actually went the opposite way: it removed the ability to limit the devices that echo sees. 
They have said that they are working on bringing some version of that back, but no details yet and no timeline.
At present, there’s no way to do it smoothly.
We had a somewhat similar situation in our house (we are three housemates) which we solved by at that time by putting a Google home mini instead of an echo in one housemate’s room and only using the native integration for the devices (Hue bridge). That way that person (and in particular a friend of that person who shall remain nameless) can’t accidentally turn off lights in other parts of the house.
But that doesn’t work if the voice integration is controlled by smartthings, because smartthings is now not only giving the voice assistants access to everything at the location, it’s giving them access to everything at all locations. 
Like I said, they have said they may change that in the future, but who knows exactly what it will do then? You can follow the conversation in the following thread: