Banning a web site on the basis of a private complaint

yes , he can file a suit for injunction and also for damages.

I think first you have to enquire the domian registrant of that website where posting made. You can make a letter to the webmaster of that website regarding complain. This is the quickest process. Webmaster can ban that person's posting. If the site is a forum based , then the site owners perhaps already put disclaimer for comment, so banning will be a problem here. Contacting webmaster through site's contact info side by side police complain is better.

Thanks,  but the problem is that those persons can post allegations by using different e-ids, isnt it?

Not at all , when the webmaster know the issue, he/she can ban or autoban any negative comment on the issue, or with the name of the busness of your client. If we provide the website name where is posting happening , i can help you better....

That's well said.

Notification of your objection through a notice to the person making the allegation and

the medium that publishes the allegation should be the first step.

But this should be done only when you are pretty sure that you are righteous and the allegation is false.

IT MIGHT BE CYBER CRIME. GO TO POLICE STATION.

MR TAPASH ALSO GIVE GOOD SUGGESTION.