Keep your code private if that's your attitude though. You don't want to share your code, you just want free advertisement.

You know what, you wrote your code, you didn't write github, you don't own github and you don't own the interaction github users have regarding public projects.

PRs are not your software. What you need to do is maintain a private git repo, or just make the repo private and post tarballs and builds on the releases page. that will solve the problem even better for you. you'll still be publishing source as well as executables that way. PRs are about github, it's users and their interaction with projects. When you give someone access to a repo, you can't complain when they create a feature branch and request a merge, that's all a PR is. In your case, you want to give people access to source code but not a repository, so do just that and leave people who want to publish a repository alone, or restrict access to your repo to a github organization with people you approve of to make contributions to private repos.