You will have to ask Bravenet staff to delete it for you. You should have deleted your site first before deleting the account as you now no longer have access yourself to delete it.
Go into your account then into web hosting and then into the file manager. Delete all of the files there to delete the master copy of your site. You can then delete your web hosting from your account. Cached copies of your site elsewhere on the internet may or may not disappear eventually.