If I'm invited to an recurring event on Google Calendar, how can I mark myself as "free" for the whole series and not only for individual events? If I click on one event in the series and edit my status to "free", this only applies to this one event? Using GApps for Enterpries on the Google Webclient in case that matters.
What you ask for Google Calendar recurring events is not a native Google Calendar functionality. I suggest you can request Google to add this via Google Support.
In Google Calendar settings you can change general setting Automatically add invitations to "No, only show invitations to which I have responded".