If I understand you right than you want to make sure that no user data are compromised even if your site gets hacked. First you should be clear that this only can apply if you detect hacks to your site. If the attacker manages to hack your site and you don't notice this then the attacker can track all activity on this site and probably de-anonymize the users which use the the hacked site.
You should also be clear that users might be de-anonymized by the way they write, i.e. the writing style, the time they write, the topics they know about and the arguments they use. Thus full anonymization will probably not be possible because the information the users inadvertently provide on themselves.
Apart from that you should never ever store any information which might be used to make de-anonymization easier. That is you should forget the e-mail address provided by the user immediately after you've sent the verification mail and should only associate some random token with the account. And you should not associate any posting by the user with the account or with each other because analysis over multiple posts by the same user provides way more information about the user than a single post would do.
Of course these restrictions in the information you store impact the usability of the site. You can also never provide some password recovery since you don't have any information about the user (no e-mail, no secret questions). You can also have nothing like all posts by the user or even recent posts because you don't associate posts with each other. And it might even be hard for a user to edit its own postings, although see this question for possible ways.
At the end you might need to find a balance between anonymization and usability. But to do this you should thoroughly analyze the situation, i.e. do an evaluation of how much anonymization you need to provide and which risks your and your users are willing to take at the cost of better usability. And of course the users must be aware about the risks, so don't claim full anonymity if you cannot provide it.