As the jabberLocalConfig.xml is encrypted in newer Jabber Versions, you cannot modify this one anymore. If you want to disable this option in Cisco Jabber globally, there is a hidden parameter in the jabber-config.xml

<Options>
<USER_ENABLED_IN_A_MEETING>false</USER_ENABLED_IN_A_MEETING>
</Options>

To find this, I had to install an old version of Cisco Jabber and change it. Now I could see in the local config file, what Jabber wrote into it. Then I could insert the parameter into the jabber-config.xml. And I was lucky, it worked. Afterwards I looked into different documentations and I could not find anything about this parameter. Let’s hope Cisco does not remove it in future realease and will update their documentation and options in CUCM12.5, where you can configure those parameters in CUCM.