Google has aligned itself with universities concerned with plagiarism by students, and has decided to ban ads for essay-writing services from appearing on "Sponsored Links". 

Users will still be able to find the ads via the main Google search feature, but they won't come up unrequested via Sponsored Links on sites that are part of the Google content network.
The search engine company has already initiated similar prohibitions for promotions for alcohol, bulk marketing tools, hacking sites, sexual content, weapons and fraudulent documents.