Automatic echo-cancellation is ON by default in VDO.Ninja.
You can turn off echo-cancellation by adding &aec=0 to a source link (guest). If you are using &proaudio, echo-cancellation gets turned off. You can enable it again with &proaudio&aec=1.
You can also switch it on or off via the audio settings. If you want the guests to be able to change the audio settings by themselves, use &mediasettings on the link of the guests, as per default only the director can change the audio settings of the guests.
May need to be disabled to use &proaudio on some older browsers.