Are you referring to Office Online, or the Office apps installed on the Hub? Office Online (eg. Word, Excel, PowerPoint, OneNote) saves whenever you make a modification even as small as a character. The Office apps installed on the Hub - I'm not so sure but I wouldn't be surprised given they are effectively the same as the mobile apps.
Save Button is old school, it is going to disapear in the long run. This behavior is changing as we speak on Office 2016 and is already in place in Office Mobile. It is not something you can disable. The reason why is we are working hard on coauthoring experience and not having to save is a prereq. As we want a seamless experience the save button is removed. Hope it makes sense