Let's assume he is someone who has been banned once..do you want him to be banned again? does he/she bothers you?
It's not about whether so-and-so bothers him, it's about respecting Bryan's decision that the person was banned in the first place.
Being banned basically means "please leave, you are not welcome here".
It does not mean "feel free to ignore our decision that you are unwelcome here".
Imagine that it is your home and you've asked someone to leave. You almost certainly had your reasons for doing so and you expect your decision to be respected. If you walk into your kitchen only to find that person back again, helping themselves to the left-over pizza, drinking your cool ones and letting the dog help himself to whatever he likes then you'd be rightly pissed off. Same thing here: "go" means "go" not "try again".
If someone wants back on the site after being banned the proper procedure is to email Bryan and request a second/third/fourth/whatever chance. FWIW Bryan often grants such requests. If he chooses not to it's usually because he has very good reasons.
Strangely those who have been banned usually don't ask for re-admittance. They often prefer to sneak back in by creating a new login name. They're basically saying "F-you, this is what I want and I'll do it regardless of what you think." This tells you something about how much they respect the site and Bryan's management decisions thereof. And it also tells you that the original ban was exactly the right thing to have done in the first place and that it should probably be re-enforced with extreme prejudice.
Nifty was perfectly right to ask Cylon's previous history on the site. It's up to Bryan to decide if the guy should be back on the site or not. If Cylon ignored that and took it upon himself to weasel back in then he deserves to be dragged out into the open and face the consequences.