Короткий путь:
Uri eventUri = Uri.parse("content://calendar/events"); // or "content://com.android.calendar/events"
Cursor cursor = contentResolver.query(eventUri, new String[]{"_id"}, "calendar_id = " + calendarId, null, null); // calendar_id can change in new versions
while(cursor.moveToNext()) {
Uri deleteUri = ContentUris.withAppendedId(eventUri, cursor.getInt(0));
contentResolver.delete(deleteUri, null, null);
}