Decouple an Echo Dot from my account

[ChaplinCharlie]

Plan

Premium

Country

USA

Device

(having same problem on both phone and PC)

Operating System

(Android AND Windows 10)

My Question or Issue

A while ago I played something through my mother-in-law's Echo Dot, and now I can't get her Dot to decouple from my Spotify account. (My login is the master, I guess; my wife and mother-in-law are "family members.") Whenever she or someone plays something on her Spotify login through her Dot, it starts playing on whatever device I am using (both my Android and on my desktop). Her music cuts off my own.

I've tried formatting her Dot as well as uninstalling/reinstalling Spotify from her phone AND my phone.

It's been enough of an issue to make me want to close my account, except my wife, who's on the family plan, would lose her playlists. If I can't remove my mother-in-law's device from my account or find another fix, can my wife at least transfer her playlists to a new account of her own?