Log on to webmail @ http://webmail.vi.net. Follow these three steps:
1) Go to OPTIONS -> DISPLAY OPTIONS -> enter '999' as the number of messages per page.
2) Go to OPTIONS -> Deleting and Moving Messages
---> untick 'When deleting messages, move them to your Trash folder
instead of marking them as deleted'.
3) Go through your mails and tick all mails that
need to be deleted, and delete. Once all selected, use the purge
deleted option to clear them forever from your mailbox.