Use some language from the content below and add it possibly to an FAQ section on the pages:
As per Google's guidelines (https://developers.google.com/open-source/gsoc/help/responsibilities#to_google), organization administrators are supposed to let Google know about the incident so that they can remove the students from the program and freeze their accounts. Google will also convey the message to them about why they were removed, so hopefully, they have some insight into it.
On some other forms of plagiarism, from an earlier thread on the GSoC mailing list:
"Now plagiarism is also apparent when students copy directly off some source and don't cite the reference, etc. This is clearly not okay either. However, after many years of dealing with this with our teenage students, most mentors have decided that it is best to teach the student that it is not acceptable to cut and paste things from a source. They have tried to give the student the benefit of the doubt the first time it happens, teaching them the correct way to cite sources. The student is removed from the program (for GCI).
As you are already seeing, there are going to be some "spammy" proposals that are one or two paragraphs pasted from some source or just not relevant to the project idea at all. Just ignore these proposals and move on; they aren't worth your time or my time. Spam happens. I am asking you all not to report these to me as I don't have the time to try and freeze all of the spam proposals. Simply ignore the proposal and review all of the relevant proposals."