How can I stop unwanted notifications from Google Calendar on an iPhone?
This is actually a default setting by Google that can be changed.  To change this setting 


Go to your Google Calendar settings found in the top right under the gear icon.
Select the Calendar tabs
Click the Shared: Edit settings link for the calendar you want to edit.
Select the Notifications tab
Click the remove link next to the reminders you don't want.


Please note that this will only fix events created from this point forward, you will still have to change all the notifications in events that were previously setup.