Click on the "Options" icon after logging in, then click the link for "Deleting and Moving Messages." On this page, there is an option that reads "When deleting messages, move them to your Trash folder instead of marking them as deleted." If you temporarily un-check this box, you should be able to delete/purge-delete any messages when over quota. The error you received was due to the fact that when deleting messagees, webmail was in fact set to copy the message to the trash folder.