industrial weed killer?

Even overlooking the possibility of permanent long term harm to yourself and, possibly, your family, you stand a good chance of having the pants sued off you by your neighbour if it kills his plants.

The courts would look very unfavourably upon a person that not only uses products they know they shouldn't use but uses them in such a way as to do harm to someone else's property and, possibly, their person.

Although it is unlikely that there is a specific law about using 'professional only' products, the use of them could be construed to be knowingly using a product that should not be used without full Health and Safety training and with insufficient Assessment of Hazard.

Not really worth doing it when proper use of retail chemicals should do the job. Someone else asked what you are trying to kill off. I'm also very interested in what it might be.