Use Akonadiconsole to configure the active alarms resource natively, and select read-only. This deletes the existing collection and creates a new one which has no KAlarmCollection or KAlarmCompatibility attributes. This makes it impossible to make the calendar usable again unless Akonadiconsole is used to delete the resource, before recreating the resource. Reproducible: Always
Ensure that a KAlarmCompatibility attribute is added to every new collection, so that if it is a compatible format, it can be recognised as a potentially writable calendar. Git commit 69dbf8e16dbab06bbc9e65017c2a2d586757a5a1 (KDE 4.12 branch).
Fixed in KDE 4.12.5 and 4.13.0.