You imported to iCal into an existing calendar (e.g. your
Home calendar), and now you have a mix of anniversaries and other events.
You want to get rid of the anniversaries without without removing your other events. You do not want to do this manually.
(1). Now you have listed all BirthdayScanner X events - and just them - in the lower pane (2).
Note that you may 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.