A productive antispam evening... 
As I've mentioned before, I've completely setup my SQL Antispam procedure and now it works good. I like this solution mainly for 2 reasons:
- If I'll install a new skin, I don't have to modify anything on my blog.
- I can completely customize what I define as "spam".
- Also a human spammer can't post anything on my Blog.
this solution however has a main disadvantage:
- this is an antispam method that involves a little modification of a .Text stored procedure, so if a day I'll update .Text to another version, I've to modify the stored again.
To keep my spam table always update with new spam definitons, I've setup a little tool that simply connects to my SQL Server and update the spam definition... useful!
When I was at the end of all this work, I had a "mail talk" with Andrea Boschin and he gaves me his CAPTCHA Image Control for testing... 
He has worked on a user control for .Text easy to use and to integrate on every skin you have (you've only to modify some lines of code in your PostComment.ascx file). I've installed it (as you can see now) and it works quite good (what I'd like to improve is the image rendering... sometimes I've see that it's quite difficult to read, expecially with Firefox).
And now? What will be my final antispam choice for my Blog? The SQL solution or the CAPTCHA Control?
For the moment I've taken this decision: I'll use the two methods together! CAPTCHA Control will require a human interaction to leave a feedback on my Blog, so spam bots are completely erased. If a spam bot (quite impossible) will pass the CAPTCHA control or if a human spammer will try to post something not admitted on my Blog, their post will be automatically deleted by my SQL control. Great... I've the most spam protected Blog of the Blogosphere!!
UPDATE: The CAPTCHA Image rendering now has a low distorsion, so the image seems more readable on every browsers.