How to override global before_script with empty before_script in job

You should use an empty array notation.

before_script:
- global before script

job:
  before_script: []
  script:
  - test

Yes, the local before_script overrides the global before_script. To keep the .yml sintax valid, use a command that does nothing.

before_script:
- global before script

job:
  before_script:
  - ''