&autogain=0
to the guest/push link in VDO.Ninja. This disables auto-gain functionality in VDO.Ninja, but it will be up to you then to manually set the gain accordingly. VDO.Ninja does offer a manual gain option that the director or publisher can control, and this gain-option does not interfere with the mic's own gain setting.
If a guest has already joined a room, and this is an issue, the director can remotely disable the guest's auto gain via the advanced options -> audio settings -> auto gain control.