In the terms of use, a GrabCAD member has the right to upload only pictures files. Then the "model" uploaded will not be visible in other place than in the user model page.
However, publish great rendered pictures with a fake 3D model or only a non representative part of the model is not allowed.
If the user doesn't reply to your message, you have a "report this model" link in the right back corner of each model page. I have already used it several times and the GrabCAD team always replied to me and have deleted the incorrect models.
You also have the same link to report a user if most of his shared models are out of rules.
In addition, use GrabCad to sell files is forbidden.