To delete an issue, you must log in as an Editor. Go to your User Home --> Editor --> Back Issues, and click the Delete link next to the issue you want to delete.
To delete an article, you will have to go to the article's submission Summary page. Find the article in either the Unassigned, In Review, In Editing or Archives lists from your Editor's page, and click on the title. Then click on the article's Summary page, scroll down until you see the option to archive the submission, and do so. You'll be prompted to send an email: either send or skip the email to properly archive the submission.
Archiving a submission removes it from the Review/Editing workflow and places it into the Archives. You can visit the archives list, and any archived articles can be permanently deleted there by clicking the Delete link.