How do I disable this and only this midi parameter?

Easy. You don't. What you need is to control the volume of your MX the Synth engine separately from the volume of the MX your audio interface.

If you are using the Yamaha Steinberg USB driver to communicate MIDI and audio between the MX and your computer, this means the volume slider of the MX is not only controlling the output of the MX, it also is acting as the audio interface for all audio concerning your computer (which includes the MX synth engine as well)

This is why we say what you need is to control the volume of the MX Synth separate from the audio interface.

The overall volume of the MX is a sum of each of the sixteen multi-timbral PARTs. Each Part has its own individual volume control
Press [PART SELECT]
Use the buttons [1]-[16] to select a Part
Cursor Down repeatedly to view the list of Part parameters. Each Part has volume, pan, etc

All sixteen Parts are summed together and controlled with the Volume parameter located here:
Press [EDIT]
Select 01: COMMON
Select 05: GENERAL
Cursor Down to VOLUME
default is 127 -this represents the contribution of the 16 MX Parts to the overall combined signal that travels OUT -separate from what is output by your computer.

it is because the MX is both a sound engine and your audio interface that this situation exists. You work around it by working with it. Each contributing sound source has its own contributing volume and all feed the main MX output. Make sense?

Hi there, it does. Thanks for pointing me in the right direction.
Using Vycro now to adjust the volumes of each part. Easy for live performances 🙂

Thanks 🙂