By hijacking, do you mean that the music is being automatically switched away? Similiar to how it is when you try to listen on multiple devices from one account?
A fix you could try, is to setup the individual Amazon Alexa Household accounts, and linking the Spotify accounts to their respective owner's Household accounts. That way, you can switch between users and allow for all of you to listen on your own devices.
Hope it helps. If it does, feel free to mark this as the solution. If it doesn't, get back to me and we can try something else. Thank you and have a nice day!
Ready to help you get back to distraction-free listening