Apple - Prevent EarPod play/pause button from activating Siri on macOS Sierra

It may be related to the fact that although you "disable" Siri, the process is still running. You can easily see that in the Activity Monitor, by searching for Siri.

If you try to kill it, it will be restarted by launchd. To stop the automatic restart, disable it by editing /System/Library/LaunchAgents/com.apple.Siri.plist and setting Disabled to false.

To do that, you first need to disable SIP (system integrity protection), edit /System/Library/LaunchAgents/com.apple.Siri.plist, then enable SIP.

Sounds like a bug in Sierra. Submit a report. For now, enable Siri and just don't use it until it is patched.