.github/workflows/do_not_merge.yml

name: Do Not Merge

on:
  pull_request:
    types: [synchronize, opened, reopened, labeled, unlabeled]

jobs:
  do-not-merge:
    if: ${{ contains(github.event.*.labels.*.name, 'DNM') }}
    name: Prevent Merging
    runs-on: ubuntu-20.04
    steps:
      - name: Check for label
        run: |
          echo "Pull request is labeled as 'DNM'"
          echo "This workflow fails so that the pull request cannot be merged"
          exit 1