I have the issue of being notified of all calendar events in shared calendars even though I think I've done everything I can not to.
First, I have some shared calendars set to sync with my iPhone, which I set up here:
https://www.google.com/calendar/syncselect
Second, I've made sure that all Reminder options are unchecked when I go to the settings of those shared calendars through a full desktop browser.
Third, under Settings > Mail, Contacts, Calendars > Calendars, I set the Default Alert Times for Events and All-Day Events to "None" just in case that helps.
Fourth and last from what I can tell my options are, in that same area, I try turning "Shared Calendar Alerts" off, but that only seems to be related to new, modified, or deleted event alerts, not for alerting prior to when shared events occur.
I want to say it's a limitation or issue with CalDAV or with how iOS handles CalDAV.