Disable Icon/user Interface

You can, but not much to save.

Thanks for your answer, how can I do it?

You disable the interface when press on the icon in the notification area (a.k.a. system tray) with the right mouse button and select Exit. You will remain protected by Bitdefender's service unless you disable it before closing the interface. Detections will be quarantined and events will be logged. The interface may be restarted through the Start Menu or the software's desktop icon.

Great, thanks! How can I prevent it to start on system start/user logon?

I checked in HKLM\Software\Microsoft\Windows\CurrentVersion\Run and HKCU\Software\Microsoft\Windows\CurrentVersion\Run and didn't find any BitDefender related keys there.

You cannot prevent the interface from starting. You will not find a key or an ini file regarding it. The interface is started automatically by the antivirus service when you log into Windows. I believe there could be other times when the interface gets started the service (possibly if there is an important message).