From Members, open the person and use Sign out: they are signed out of the organisation on every device at once. They can sign in again, so if you want to keep them out, suspend them instead. Every sign-in and sign-out is in the audit log, so filtering on auth.login and auth.logout tells you who used the organisation and from where.
For the shared PCs: tell people not to tick remember me there, and to sign out when they leave.