Login to the Admin Suite.
Navigate to Setup Tab -> Extensions -> Manage
Choose extension to restrict and click Modify Button.
Navigate to Permissions Tab.
Expand 'Use PBX Features'
Disable 'Use Switchboard Chat Panel'
Click Save SIP Extension
Login to the Admin Suite.
Navigate to Setup Tab -> Extensions -> Manage
Choose extension to restrict and click Modify Button.
Navigate to Permissions Tab.
Expand 'Use PBX Features'
Disable 'Use Switchboard Chat Panel'
Click Save SIP Extension