@Holgi Now I’m confused because I have read “address book” in your title, but now we’re talking about calendars.
Is it about address books or calendars?
Concerning calendars, DAVdroid already sets read-only CalDAV calendars (which are shown as read-only in DAVdroid) to read-only in the Android calendar storage, so calendar apps can’t make changes (with works with the tested calendar apps).
Address books and calendars are implemented very differently in Android and are not related to each other. Can you please provide verbose steps to reproduce your problem (including all involved software and details)?