HI Bhavesh,
Greetings!!
You can use disConnect API to disconnect user from AppWarp on back button event.But it will disConenct the respective user.
But If you want to disable private chat option only without disConnecting the user, in this case you need to write own businees login. You can manage a flag in Shared Pref and can enable the private Chat mode accordingly.
Let me know if it helps.
Thanks
Vishnu Garg