The hardware MIDI THRU is hard wired to the MIDI INPUT so anything received on physical 5-pin MIDI INPUT will appear on physical 5-pin MIDI THRU. That is the point of MIDI THRU.
If you want to stop MIDI INPUT appearing on MIDI OUTPUT, that is a different question and may be possible.
Thanks again Riban for your prompt and quick reply. I connected my kawai mp10 via midi cable to zynthian (in and out) and via midi thru port (zynthian) to a roland expander. This way both zynthian and expander play simultaneously. I would like to be able to disable, without resorting to configuration changes, the sending of note on messages from the zynthian device to the expander.
Hi, I’m just currious, what’s your use case of zynthian’s midi out to your kawai piano midi in ?
Sorry but I don’t think I understand exactly what you are interested in knowing specifically.
You Said that zynthian’s midi out is connected to your electric piano midi in plug.
What kind of midi message do you send from zynthian to the piano ?
There is no specific reason … let’s say that due to atavistic habit (linked to the old system of data transfer of sounds using BULK DATA) I connect the keyboard and expander in Handshake mode.
It’s not an area that had much of a look, and I find it interesting that the Nord Modular’s ( a commercial device somewhat like the zynth) never did too much with it either . .
Baiscally it’s a hard problem.
The obvious response it just echo input, and leave it at that. But the next stages get complicated very quickly. . .
How do you select the source you wish to mix into the outgoing stream? and more importantly allocate the channel information to it, because you are that becomes a big problem very quickly with lots of recursion.
Not something it’s easy to config with 4 encoders and 4 switches !! 
So we will perhaps, turn the question round, and ask what would you like to do with it.?
IN some techy world they are known as Use Cases and they help identify specific needs.
Pointing one MIDI source at a separate specific device via a zynthian. Is an interesting use case.
Using a Zynthian as a MIDI patch panel. (Cos we have to think about someone plugging a MIDI interface into the zynth with maybe 8 or more 5 pin DIN sockets …) is an excellent example of a use case, and a touchscreen interface to that would be very intuitive, but the GUI implementation would be less easily done and would require a fair bit of opinion. And then there’s a the actual coding of it. . .
But we often run into these sorts of questions from other areas of development. It’s likely that the zynseq project look outwards to control all manner of wierd and wonderful external MIDI devices so any observations from a defined Use Case that already exists, provides insight at precisely the stage of a project where it can be of the most help.
We keep useCases in the
Zynthian Issue Tracker . .
As feature requests.
Well that’s how it’s meant to work ! 
Oh yeah, I would use this!