I found a work that violates pixiv's Terms of Use or Guidelines

If you find content that is forbidden from posting as described in the user agreement or guidelines, please report it to us by clicking on “Report a problem” on the lower right of the content screen. After we verify your report, we will address the corresponding content and the poster of the content.
Please refrain from supplying information that is unclear, ambiguous, or fabricated.

Service Master Terms of Use
pixiv Guidelines