Users would have to request deletion and some sort of proof that the account/email was theirs, how they do that I don't know. As an Avast User helping other avast users I'm limited in what I can do. I guess it would have to be a request to one of the Avast forum Administrators.
As the account essentially doesn't exist, if you click on a username you would see an error. It isn't a case of deleting the account and posts in one action. Posts would have to be removed manually.