I’m not at all sure I follow what you are describing... but MONTAGE Keyboard generates Aftertouch — it a sensor that runs underneath all the keys... any key can be used to generate the Aftertouch message. It is not possible to zone the Aftertouch on the MONTAGE within a single Part.
But since you can transmit to multiple Parts simultaneously using the KBD CTRL feature, you can program a situation where you copy the sound to an adjacent Part; set one Part to respond to AT and the adjacent Part does not... set each to cover a different range across the keyboard. In this fashion you can zone (Note Limit) the Aftertouch.
The type of Aftertouch MONTAGE generates is called “Channel Aftertouch”.
Is there some way to stop AfterTouch messages from being sent to Cubase?
No, not how it works. Like most controllers they always generate their message, it is on the receiving device to either do something or do nothing in response to the message. Like the PB Wheel always sends its message, the receiving device determines what if anything happens in response.
Cubase has a ‘Receive Switch’, of sorts!
In Cubase, go to Cubase > Preferences > MIDI > MIDI FILTER > here you can filter Channel Aftertouch from being Recorded and Thru’d back through the system.
Place a check mark next to Aftertouch.
I prefer to always Filter AT. Only when I actually need it, do I activate it. I use it so rarely it makes sense to filter it by default... that’s why it’s a “preference”.
Yes, it's easy to activate aftertouch in normal playing and the extra MIDI messages is more extraneous information to deal with if not filtered. Putting an aftertouch "firewall" on the input side of Cubase makes sense as a default "template" for how BM uses the instrument.