There's a sysconfig to turn off nearly everything in OS X if you're comfy with the CLI, I doubt Siri would be any different. It will probably be a usual GUI switch though, due to privacy laws.
I was pretty disappointed by this WWDC. Any info on next XCode version?
Before you decide you're disappointed, wait for the Developer State of the Union later today and the other sessions. There's interesting things like an "Apple File System" session. It looks like the beta for the file system will be in macOS Sierra and come out officially in 2017. You won't be able to use it as a boot drive (probably a safe precaution) and it has a few other restrictions, but it's a good start.