Edit as root:
/usr/share/pulseaudio/alsa-mixer/paths/analog-input-internal-mic.conf
/usr/share/pulseaudio/alsa-mixer/paths/analog-input-mic.conf
Search "[Element Internal Mic Boost]" set "volume" to "zero".
Search "[Element Int Mic Boost]" set "volume" to "zero".
Search "[Element Mic Boost]" set "volume" to "zero".