A spammer Hijacked my client's google listing! how do we fix and prevent this from happening?

I was troubleshooting a problem all morning where a spammer hijacked a client's google listing, using all his header tag content yet making the title of the top google listing, a link to an unseemly ad page. So people using keyphrases relevant to the client's website see his info in the top search engine returns, (description), click on the link and are redirected to spam pages including gambling and porn etc. It is maddening! The clients actual URL is listed in the google listing but not as a live link, so unless someone knew to type in his actual URL they are deceived.

I tracked down the spammer service to a site called changeip.com, tho of course they don't provide legitimate contact info. I'm working to get the registrar of the spam ad page domain to shut down the domain. In so doing, the registrar rep realized the registrant has a wholesale account loaded with many domains that unknowing people are being re- directed to in the search engine listings, by way of the same deceptive means.

If you know anything about how to prevent this from happening or about how to fix it, I urgently need to find out!

This is scary stuff that I need to learn to avoid and fix!