Disable Left/Right Scrolling on Magic Mouse with Google Calendar
There are tons of very old threads on the Google Forums around this but I finally turned up one which linked to this Chrome plugin which kills the scrolling in Google Calendar. Phew!

https://chrome.google.com/webstore/detail/google-calendar-scroll-di/nghndfiaocgpmcbeafglhknklfgddebe

As a side note, here's an interesting bit of debugging by Peter Torelli which clearly shows it's just lazy coding in GCal: https://productforums.google.com/forum/#!topic/docs/HeZb8Wv8X5k