To remove “Reported attack site” banner from the website, website should be checked for Malware or malicious links.
The following sites are helpful in identifying the malware in a site.
Still the domain needs to be checked with the help of a programmer to remove it thoroughly. Once it is cleared, we can report to Google webmaster tools to remove the listing (or to remove the banner from the site). Also it is highly recommended to change the existing passwords of the domain.