I don’t know if you have ever seen a warning like this when visiting a website.

These sites containing Malaware are reported everyday from computer users like you. If you install a program and it contains malaware,spyware,virus and such, you can report the website through different websites.
Badware websites
A badware website facilitates the distribution of badware, either intentionally or because it has been compromised. Many, perhaps even most, badware websites distribute badware without the knowledge of the sites’ owners.
Drive-by downloads occur when a website automatically (and often silently) installs software when you visit the site. Typically such an attack takes advantage of a vulnerability or “hole” in your web browser, a browser plug-in, or other software on your computer.
Social engineering attacks take advantage of human nature by tricking people into installing badware. One common ploy includes showing a fake virus scan that indicates that your computer is infected and encourages you to download and/or purchase a tool to remove the infection. Another common trick is offering to display a video that sounds interesting, but only after you install a plug-in or codec that is “required” to view the content.
You can report a bad website by visiting https://badwarebusters.org/community/submit
and you can verify if a website have been flagged by other user at http://www.stopbadware.org/home/reportsearch