IOS – way to lock your Apple device against syncing on other computers

From my understanding you decide at https://www.google.com/calendar/syncselect which calendars are to be synced with CalDav devices. No more, no less.

On the devices themself you can choose which calendars you want them to displayed. (It know this is quite trivial, but I do not quite see why this does not suffice).

Alternatively I would suggest creating mutiple Google accounts each one with its own CalDav syncing settings. For each one you can decide which calendars are to be shared via CalDav. Each of your devices would be connected with one of these Google Account. Then you share calendars among the Google accounts.

E.g.: Lets say A has all the calendars. You have an iPad and an iPhone. For each one create a google account (B and C). Now A shares the iPad related calendars with B and the iPhone related calendars with C. The iPad syncs with B, the iPhone with C.

It is not ideal but once set up it should run without any overhead.

Erase all content and settings does exactly that, it erases user data and restores default settings - but leaves the current version of iOS intact.

Performing a full restore through iTunes is the only way to completely erase the device and perform a fresh installation of iOS.

If you've any doubts then the iTunes restore is probably your best option as it will update your version of iOS, with the option of backing up and subsequently restoring your apps, music and other settings with the minimum of effort.