You can view the active sessions in Salesforce and kill suspicious sessions if required. Navigate to Setup -> Security Controls -> Session Management. Select one or more sessions and click on Remove to end the suspicious sessions. Do not forget to freeze the user account (Setup -> Manage Users – Users) or reset their password if you suspect a compromise in the user account

Kill Sessions in Salesforce From Session Management

(This blog was originally published in May 2014. The screenshot has now been updated with Lightning UI)