Cause when user is baned - we point him the reason and it is described on help/terms page.
But there is no chance for him to see it been baned or maybe i'm wrong?
Ok, but there is also no chance to logout to use "contact us" or just maybe read forum pages (clear cookies etc.)
Banned user has no chance to be a user any more - ok. But imagine that you make a mistake and ban wrong user or two... What should those users do to indicate an error happening?
This system has no responsiveness and a chance to correct what was done wrong
It looks like "Any feedback, anyone? " ...
All valid websites have a list of emails that should be setup when the site is open to the public. webmaster@domain.com is one of them. As for spam, well your email server should be setup to deal with spam. Personally, I use gmail to access most of my private emails such as that. So setting up a decent spam filter is rather easy.
If you want your users to know about the email, you could change the default message that is emailed to a user when they're banned and make sure that you check the box to send them the email when you ban someone.