On the website: Click your avatar (top left), then select Settings. In the settings find the Security part and there you have the buttons to end sessions (force logout). Note: This option only works for the website on Browser/Phone base, not on the game itself.
If you think your account has been stolen I highly suggest you read through this wiki article: wiki/en/Help_centre/Account#account-theft (Sign-in > My account has been stolen!)
Further, I'd suggest to also change your password on the end of you email service, in case there has been a security breach.