You imported into an existing calendar (e.g. your Home
calendar), and now you have a mix of anniversaries and other events.
You don't like that and want to get rid of the anniversaries without without affecting your other events.
zencat
(1). Now you have listed all BirthdayScanner X events - and just them - in the lower pane (2).
You might have to repeat that whole process (including step 2, by re-entering the search term) multiple times. iCal seems to be a bit buggy and only deletes parts of the selection.