It's often very hard to trace back everything that got published from a given publication.
One easy solution can be done in the backend (note: this may not be supported by Tridion and could void your warranty).
1) Grab the publication ID:
a) through the interface by mousing-over a publication and grab the highlighted number tcm:xx-XX-xx
b) through the backend with a simple: select * from publications and find the ID for the publication you'd like to delete.
2) Now you can delete the publication starting with dependencies first:
delete from ITEM_STATES where PUBLICATION_ID = XX
delete from PUBLICATION_ITEM_ASSOCIATIONS where PUBLICATION_ID = XX
delete from PUBLISH_TRANSACTIONS where PUBLICATION_ID = XX
delete from TRUSTEE_RIGHTS where PUBLICATION_ID = XX
delete from PUBLICATION_PRIORITIES where CHILD_PUBLICATION_ID = XX
delete from PUBLICATIONS where ID = XX