For example, in one project, PRs have to be submitted to the "next" branch and not the default branch. This is written in the CONTRIBUTING.md file, which is linked in the PR template, with the mention that PRs that don't respect that will be close. Most if not all submitters of low-quality PRs don't do anything once their initial PR is closed.
Pretty bummed about that as I just submitted a show HN I'm pretty happy about (it solves an annoying problem I had for years, which I know many people have) and I was looking forward to talk about it (https://news.ycombinator.com/item?id=47050872)