Stop the audio signal between your host and MODX in what direction? From MODX to the PC (MODX output), from the PC to MODX (MODX input), or both?

Your MIDI routing makes sure when your MODX piano keys are pressed - the piano keys are disconnected from MODX's tone generator. Therefore, MODX's tone generator will not make noise unless there is a path for MIDI to circle back from MODX's output back to MODX's input.

This setup would be inside your DAW's MIDI track (not shown).

For audio, there's missing audio routing inside Cubase how you have the audio bus set up. If you do not want the DAW to send audio through the USB bus back to MODX - then you would remove MODX from the audio chain through your audio routing.

The default is to have your system (PC) audio routed to Main L&R. This is a Windows noise (like youtube and beeps and such) setup. You can either mute this or remove MODX from your system sound outputs.

What you've presented is an incomplete picture of your setup - which may be part of the problem. Not understanding where to find these and set them.

The other thing missing is how your MODX connects it's MAIN L&R analog outputs (1/4" jacks in back) to either speakers, the RME, or nothing. You mention the FA6 and Kronos are connected to RME - but it's not clear how the audio for MODX is connected.

You do say the 5-pin DIN cable (assuming zero cables) is/are not connected to MODX. So I am assuming the MIDI IN and MIDI OUT 5-pin ports are empty.

Hello I want to know is there any way to stop to audio signal in USB to host?

It is very easy: simply do not select the “Yamaha Steinberg USB Driver” as your ASIO Driver... we can see you have selected a different Audio Streaming Input Output (ASIO) Driver... “ASIO Fireface” which I assume is for your RME.

The doubling you are hearing has nothing to do with USB Audio coming from the MODX, you have not selected the Yamaha Steinberg USB Driver, so only the MIDI portion of the Driver is being used. The doubling is caused because you setup improperly to receive MIDI from the MODX. When playing the MODX, it is important to recognize that it is quite normal for the MODX to transmit each Part on its own MIDI Channel, this requires a specific transmit/receive setup.

I highly recommend you start by working with Single Part programs. Learn to record a single MODX Part to Cubase.
Select a Single Part
Create a single MIDI Track
Record and playback.

We call this starting at the shallow end of the pool. If you start by trying to record a Multi Part program, before learning how to record and playback single Parts... it is like jumping into the deep end of the pool before you have learned to swim. Multi Part Performances transmit on multiple MIDI Channels... if you’re have selected a Performance with multiple Parts you are transmitting on as many MIDI channels.

Start with just a Single Part. Learn how it works. Get comfortable recording and playing back Single Parts, one at a time.

Also what version of Cubase are you using, Cubase AI or Cubase Pro?
If you are using Cubase Pro, for example, you can use a specific Recording Template that makes this setup straight forward.
If you are using Cubase AI (or any of the other versions) you need to setup for Multi Part programs a bit differently...

Let us know which Cubase you are working with...
Let us know which Performance (or at least how many Parts you are attempting to record).

Thank you the matter has been resolved . Actually I had not properly inspected the checklist . now I understand this keyboard has two local switches one is in the main midi-settings and the second one is in performance mode . I had turned off the first one but did not turn off the second one . The issue has been resolved.