Hi Laurence,
What is Google DCMA? - This relates to requesting Google to remove links/website from search index etc for violations of copyright or law... Check out the [URLnf="https://support.google.com/legal/answer/3110420?hl=en"]G's legal removal request[/URLnf] info page. I also came across this [URLnf="http://searchengineland.com/google-dmca-notices-reach-record-high-7-8-million-requests-filed-one-week-201327"]interesting article[/URLnf] - outlining that G got more than 7.8 Million link removal requests in one week!
How to report an issue with DCMA? In the extreme case you would start with hiring a lawyer... On the smaller end you could write your own notice with using online templates as that may stop it before you have to spend money on legal support.. Have a read of this with getting [URLnf="https://nppa.org/page/5617"]started[/URLnf] and for creating your own requests you could use something like [URLnf="http://www.ipwatchdog.com/2009/07/06/sample-dmca-take-down-letter/id=4501/"]this[/URLnf]..
and how to avoid it for your website? Well don't violate copyright e.g. images, content, software etc....
Hope that helps!
Cheers,
Sid