Firstly they threatened him with legal action not violence and this stuff about terorists isnt going to help this guys nerve if he has to confront them again. I may be wrong here but isnt it only slanderous if it is written, e.g. a flyer. Also I dont think you mentioning the disadvantages of traditional methods as supose to WFP is slander in any shape or form written or not.
When I here there is a window cleaner charging half what I charge, I tell my custies that I think they are likly unlicenced and not running a ligitimate business. That isn't slander it my opinion, its only if I state something like that as fact like joe bloggs window cleaning are brew boys(fictional, hope nobody has that name).
And finally my advice, report the incident to the police as it wil only help your case of it does continue and I suggest that you stand your ground and remind them that you have right to earn a living, and the public has the right to a compeditive market.
Finally I wouldnt suggest it only if its a last resort, but if you carry a glass scrapper you do have a wepon to defend yourself if cornered but I would concider the fact that they are window cleaners they may also have.
The last thing you want is for it to result in vilolence, but I doubt it will get to that. You might want to concider learning who all these people are, just go up shake there hand and introduce yourself if you see them, ask for there number and tell them you will pass any work to them you cant do. Then if anything happens you know your enimies well, also take a pic with your phone of there car, and make sure you get there reg. This could be important info to pass to the police and could help get a confiction if they try anything else.
That is the only way to hurt them, if where you are you require a licence to work as a window cleaner, they are less likely to resort to violence as they know if they are charged and convicted they may not get another licence, and be out of a job.
Secondly you are doing everthing right by the sounds of things if they are panicing that much about lossing customers to you. So dont change a thing, they are thugs that wont last.