This has happened to me a few times now. Spotify Connect will connect to another users device and stream music. They are able to follow/unfollow playlists and artists. I am currently using version 184.108.40.206.gf959d4ce .
Login to your account in the website
Check that your email address is correct (not been altered by someone else) and change the password to a more secure one.
Then, there is option to sign out from all devices in your account's main control panel (at the very bottom).
That should logout of everything, including other people's devices.