It differs among ISPs, but each time your router connects to the Intenet it may be given a different IP address by your ISP, so you can't block purely on IP address to stop a specific poster. And even if you could, that poster could use a smartphone or tablet using 3G to post, again giving you another IP address each time you connect. Very difficult to protect yourself against from MCNs perspective, but not impossible.
That said, they need to sort something out, as clearly some people are more offended than others. Often I find the views on the forum expressed by punters are more interesting than the actual articles posted by MCN. Sure, some posters post in bad taste, but live and let live I say, you don't have to read the posts and you can choose to ignore them. By far the most annoying posts on the forum are spam, but you can ignore these too, i do.
I'm not saying it should be a free for all, the nutters taking over the asylum etc, but every one of us has an opinion and is entitled to voice it, no matter what their preference.
So try and play nice children, no being mean to other posters and no telling tales, or you'll be grounded this week with no pocket money, ok? Good.