Hello Guy,
Welcome to the world of MIR 3D! We hope that you enjoy it as much as we do. 😊
You're asking a valid question that has a number of possible answers.
If you just want to turn off MIR 3D's impulse responses (i.e. "the reverb") without losing the volume settings and the already implemented positioning of the direct signals, then there is indeed a dedicated button in the Venue Control panel at the bottom of the right panel section:
These buttons work separately for each Venue Tab, so that's something to keep in mind.
If you want to turn off MIR's features altogether, please use the Bypass buttons of the individual instrument headers in the panel on the left. When you keep the Alt(Windows)/Option(Mac)-key pressed, you can bypass all selected instances with a single click:
... again, this is something that works for the active Venue Tab only.
For an instant Bypass of all instances in all Tabs it might be handy to assign all Instruments to a MIR Group (F8) that uses "Bypass" as its only common feature:
... depending on the host, bypassing might give you unexpected results in the signal's routing within the host's mixer, especially in a multi channel context.
In this context it's important to understand that technically MIR 3D is a collection of individual plug-ins that "only" share a common main GUI. If you really looking for a way to switch off (not just bypass) all instances of MIR 3D, this has to be done in/by your host. Some DAWs (e.g. Pro Tools) offer a way to de-activate the plug-ins in a certain insert slot across all selected tracks: This means that you would have to make sure that all MIR 3D plug-ins use slot 3, for example, then you could indeed switch off all MIR instances with a single click. - The sad news is that this doesn't work in Cubase/Nuendo, as these applications don't allow for the deactivation of multiple plug-ins simultaneously, only for bypassing.
Please don't hesitate to ask if further questions arise!
Kind regards,