[Logician]

One of the reasons of register requirement is to check if user entered a valid address of his own.

In your system users can deliberately or accidently enter email address of someone else and you have no control on the issue. This also proves banning unusable because the address you ban might belong to someone else.

So this would be possible without email feature. You can set guest permissions to post in certain forums then apply a one line hack to put these posts into moderation if posted by a guest and in forum X. If you are interested, this hack has been released in many threads here in request forum..