If he is actively poaching your work, and even saying that he is taking over your round, then write (or get someone to do it for you) a very profesional looking letter thats spells out what is happening. Deliver this to all of your customers, but don't name him! And be careful with the wording.
Don't let any anger show in what you put on paper, be factual, point out the pitfalls of this type of unprofesional conduct.
Point out your prices are competitive and fair, that you are regular and have been going for a long time (however long that may be)
You are highly unlikely to lose more than a handful of accounts.
Also; Don't have any qualms about informing the DHSS, if he's a dole cheat he deserves to be shopped.
If you actually catch him working then have a word with him, point out that you don't mind competition and tell in no uncertain fashion that you will be an unhappy bunny if he aproaches your customers again and tells them that he is taking over your round
Leave out the violence, that can backfire on you, a brawling window cleaner is not going to enamour to any customers you have who get to hear of it
Regards,
Ian