The first step is to send a cease and desist
letter requesting immediate removal of the content. I'd sent it certified, return receipt, so you'd start accumulating evidence.
Make sure you print out his site, again for evidentiary reasons.
If the forum is run by a different company (not his own server), you may be able to ask them to have the content removed as well.
If that doesn't work, you might want to consider suing. If the statement made are false and they cause you damages, you may sue for defamation
You'd typically have to prove the following:
1. That [name of defendant] made [one or more of] the
statement(s) to [a person/persons] other than [name of
2. That [this person/these people] reasonably understood that
the statement(s) [was/were] about [name of plaintiff];
3. That because of the facts and circumstances known to the
[listener(s)/reader(s)] of the statement(s), [it/they] tended to
injure [name of plaintiff] in [his/her] occupation [or to
expose [him/her] to hatred, contempt, ridicule, or shame]
[or to discourage others from associating or dealing with
4. That [name of defendant] failed to use reasonable care to
determine the truth or falsity of the statement(s);
5. That [name of plaintiff] suffered harm to [his/her]
property, business, profession, or occupation [including
money spent as a result of the statement(s)]; and
6. That the statement(s) [was/were] a substantial
You may also consider suing for interference with business relations together with defamation.
If you decide to sue, you'd be best served by being represented by an attorney who is familiar with these types of cases.
Please let me know if I can be of further help.
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