This was also a huge obnoxious issue when I think GitHub was offering some kind of promotional T-shirt for a certain number of open source related pull requests.
There needs to be a way to setup a filter that auto denies comment only based PRs, unless it comes from a dev with a certain minimum "reputation / contribution" threshold score.
Here's that YouTube tutorial: https://www.youtube.com/watch?v=Ez8F0nW6S-w
Starting at around 1:12:00 they fork express.js and show how to make a pull request.