Could you try a couple things and then let me know if the problem still continues?
1) On Spotify.com, go to your Account page and change your password. While some users have said that this hasn't fixed their problem, it has worked in other cases. (Facebook users have to change their Facebook password instead.)
2) Also on your Account page, there is an option that says "Sign Out Everywhere" which will disconnect you from all active devices. If you do this as well as change your password then the problem should disappear.
3) Another option is to go to the Apps tab on your Account page, here you can revoke permission from apps that want to access your Spotify account. If you see anything here that you don't trust, remove it from the list. I currently have 3 apps connected.