This is how to report a website to Google.
- Go to https://www.google.com/webmasters/tools/spamreport
- Click on the option most suitable for your situation. In your case, it'd be link schemes (paid links).
- Fill out you the website which is breaking Google's terms, along with the service which is brokering the links.
It should be noted, personally, I have never had any positive results from reporting (blatant) blackhat SEO from competitors, or breach of terms on Google Ads either. I'm fairly convinced nobody reads the reports, and if they do, they don't take them all that seriously - in fairness, I bet there's a lot of petty reports when competitors are concerned. You may have to wait - Google will find what they're doing and give them a swift algorithmic penalty. It's just time.