Hi,
You should be able to select where the sound is playing by clicking where it says about your avilable devices. If you're sharing a single account, this is an issue that isn't really surmountable. I recommend you check out the family plan that spotify offers if you're sharing an account. Also remember that Spotify offers a free account! So people sharing an account don't have to pay anything to get access to all that music for free without the disruption caused by sharing a single account.
If you want to deactive other devices remotely, search in your browser for your spotify account, log in and click on 'Offline Devices' Tab on the left. You can remove devices here. This will stop you being able to play through those devices. But will also log you out of those devices and remove all downlaoded offline content.
If you're refering to streaming devices like a bluetooth receiver or WiFi receiver (like Google's Chromecast), these can only be deactivated by dsiconnecting yourself, or them from the network you're sharing. However, you're not obgligated to stream through them, just select your phone from the devices list and it should play your music through that.
Hope this helps.
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