Hi Ho,
test the ITSMobile service on the device itself.
Behaviour may not be the same as the desktop browser. If you wish to logoff completly. Then check if you have the configuration of the note
1980475 - ITSMobile - Configuration settings regarding Logon and Logoff
You are correct re "Deactivate for Service" for an individual service, this is the same as setting icf_user_check 0
See also the note
194724 Setting secure session restriction for a specific ICF service
the section: Configuration settings for systems with HTTP Security Sessions
Regards,
Oisin