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If you notice suspicious activity on your account, I highly recommend following all the steps provided in this support article.
I also highly recommend reading this support article regarding how to protect your Spotify account as well.
As soon as you have gotten your account secured, you can follow the steps listed here to remove these Spotify users from your Premium Family plan.
I hope this helps, and let me know if you have any questions.
Have a good day!
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Note: I'm not a Spotify employee.