You can't delete an account on Helium. They keep your articles forever, even if you decide to leave - or even if you're banned! However if you want to dissociate yourself from the account, you can change your user name and remove your profile image.
You can also write anything you like on your profile page and use the Links section to direct visitors to your new location.
As for other sites, they're all different. You'll need to be more specific.