Under the Criminal Damage Act 1971, the applicable definition is: "A person who without lawful excuse destroys or damages any property belonging to another int

Under the Criminal Damage Act 1971, the applicable definition is: "A person who without lawful excuse destroys or damages any property belonging to another intending to destroy or damage any such property or being reckless as to whether any such property would be destroyed or damaged shall be guilty of an offence." It's not entirely clear that the "grammar vigilante" has actually committed an offence. There is a reasonable argument to be made that he is effecting a repair rather than damage. Consider a "cleanliness vigilante" who cleans dirty windows or a "bicycle repair vigilante" who repairs parked bicycles.