Put this key in the com.apple.keyboard.plist in the /User/Library/Preferences folder and reboot: DictationOnSpaceKey false
Make your com.apple.keyboard.plist look like the one in this post: https://forums.macrumors.com/posts/14208754/
Is there any way to do this on a non-jailbroken iPhone?
I'm assuming that editing that .plist needs a jailbroken phone as I cannot find it within iTunes or anything.
I want to disable the keyboard dictation button but not Siri entirely.
You might be able to use iFunBox to access that directory and edit the file. Reboot after editing and it should take.