I won't get in to how childish the notion is but you'd need something custom coded. So unless you wish to throw money at it, you could always ask someone to code it for you, because I doubt anyone would do it for free.
I'm curious as to what he's doing for you to want to make him invisible, but not bother just banning him (I'm nosy like that!). Have your members reported him? Are they making complaints? If so, go directly to ban, do not pass go, do not let him collect £200
He hasn't violated any rule on the forum but it's absolutely annoying with useless post, polemical on everything and so on. Since he doesn't violate any rule I can't simply ban him so I've thought about make him ignored by everyone
If he doesn't pay to be at your forum you have nothing that can prevent you from banning him.
Since useless is an opinion really it would be pretty nasty to do that and potentially waste someones time and energy especially if THEY think they are doing something worthwhile.
If you don't like him, and he is not owed anything just treat him equally as a human and ban him from your personal property instead of looking down and essentially wasting someone else's time. Doing what you are suggesting is about = to being a full time professional troll to be quite honest and trolling people in a professional sense can cause you some problems.
Have you had complaints from other members about this person?
Have you messaged him to politely tell him that his behaviour is annoying to you (and others)? If you haven't, then really you should do that first to give the member the opportunity to modify his behaviour. If after that he continues in the same way then apply sanctions ranging from moderating all their posts right up to banning if necessary. IMO this is the correct way of dealing with this situation.
He said he hasn't broken the rules and he is annoying...I find many people annoying...do you think I can go to McDonalds and have a cashier fired because I found his or her mannerisms annoying...no...that's a personal thing...but..I could always have 10 of my friends go in and complain to the manager and then they will get fired and that's why complaints from other members shouldn't even be gauged unless he is breaking rules.
If you're taking money from him, then I think it's just wrong to abuse his account in the manner that you're trying to, especially as he's not breaking any rules or been informed on the things he's doing that are irritating others.