1. ワークフロー全体を無効化する

on.<push|pull_request|pull_request_target|workflow_run>.branches-ignore

on:
  push:
    branches-ignore: ['**']

2. ジョブを無効化する

jobs.<job_id>.if

jobs:
  test:
    if: false

jobs.<job_id>.steps[*].if

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - run: echo 'Hello, world!'
        if: false