Modify the internal microphone config file at `/usr/share/pulseaudio/alsa-mixer/paths/analog-input-internal-mic.conf` and add the following lines:
[Jack Headphone Mic]
required-any = any
state.plugged = no
This will disable the internal microphone from being accessible when a headset microphone is plugged in. Most likely will need to make this modification again after a pulseaudio update.