It's a few months late but I just had the same issue on my Nexus 6p. If you're still having the issue and you're rooted I have a solution.
First thing yr gonna have to do is download this app scanCode from your selfie stick.
Run the app, press the button on your selfie stick, and it should show a scanCodein there. Mine was 582
You have to edit one or more files in /system/usr/keylayout. The first file is Generic.kl, you will have to search through all the files in that folder to find other instances of your scanCode. I also found 582 in Vendor_046d_Product_b501.kl.
In both files search for your scanCode. After that should be some other instruction that is causing your phone to do the voice search, change that to VOLUME_UP
In mine it looked like this in both files
key 582 VOLUME_UP
When you're done remember to save the files, and make sure its done in all files you it was found in. It won't work otherwise. Reboot and you should be good.