Hello,
BOM delete is not so simple... If ECN is active(Even if you are not using chnage number), You have to delete date from which componenet was effective.
Go to CS15 & find out parents for material you are trying to delete... Then for those parents execute CS03 by take out From & To dates... System will show all entries that you are seeing in STPO... Note the date from which material is effective & use that date in CS02 to delete that material.
you should be able to delete material after..
Regards
Jatin