Well, I spent a lot of time analyzing the log of the site and I realized that our site has been "registered" by some sort of software used by comment spammers (like hundreds of other site like PF). I think that this software fills the registration form automatically, that's not surprising considering that I turned off the visual confirmation because some users found it difficult to understand , then when the spammer has access to the site some automatic script post the spam comment in the first forum. Very rare are the "human" spammers that take the time to register and comment manually.

However last week I added a mandatory , dull question, in the registration form (it asks: "Are you a human ? "   ), thinking that his could be enough to stop at least temporarily this automatic program or script, and actually I had no more spammers trying to register... In this week I noticed a sort of perl script libwww-perl/5.805 or with another number coming to the site (I disabled and automatically banned the perl scripts) and I think that could be this automatic registration script. I'll change that dull question in the registration page after some time , changing the postition and the possible replies. if this sytem works i could even go back to the simple activation by email instead of the admin activation...

Sounds almost too easy  
But worth a try