Long time no see, Yuri. Welcome back.
This is really the same topic as
this one.
I think Gron's statement answers this question pretty well.
Won't it spoil it for the visitors who doesn't want to register?
Quite a few people are so terrified of the net they won't even give their e-mail address for registering purposes.
[edit] Also, it appears that our spammer is using an IP hiding software. Running an IP check revealed about half of them to be scrambled and half to be Corsair's, so forcing guests to register in order to post isn't going to solve our spam issue in this case. As long as I can't find out this spammer's true IP address, I can't ban him. I tried doing that yesterday but I wound up banning Corsair instead.
However, I will not rest until this spammer is brought to justice.