This is your hedge and he is not entitled to cut any of it other than any branches which overhang the boundary. Even so, he is not entitled to keep the clippings!
If the hedge is over 2 m high can take you to court (the local authority will actually deal with it) to make you cut the hedge down to 2 m. Assuming that its under 2 m and that he hacks more than up to the boundary or any height at all then he commits criminal damage which is a police matter.
Even if the hedge is over 2 m and he cuts it down to 2 m he still commits a criminal offence of criminal damage. It is not compulsory to have a hedge no higher than 2 m but it can be made in the right circumstances to be cut down to 2 m but that is up to the court to order that subject to a person making the correct application.
Tell him that if he cuts the hedge again, it will be classed as criminal damage that you will refer the matter to the police. Put it in writing. That way, in future, if he cuts the hedge again, you can show criminal intent by virtue of the letter which he has ignored. You might also want to get a solicitor to write to him stating the same and also threatening a court application for an injunction to make him stop cutting the hedge if he will not agree to leave it alone. The letter from the solicitor can also say that if that action is necessary you will also make an application to the court for the substantial costs of applying for the injunction.
That should be the end of the matter if the neighbour has any sense.
Can I clarify anything for you?