Why can't the guy enter another email? Since you have both Captcha and Turing test installed, nothing will stop the guy from posting junk comments.

The only thing you can do is disable visitor comments.

He can use another email, but having his IP blocked, he will have to renew it to see the blog, and then, when he gets to see the blog, he will find his email blocked. It won't deter him from using another mail, that's true, but since he is proud of using his fake email, it will slow him from posting. All this in this particular case.
In other cases, I may want to block a real email, but not its domain, would that be possible?
Disabling comments is too much since many others will suffer consequences because of this person.

back to the question... did you try blocking the email on Tools > Antispam tab? I mean the same way you blacklist keywords.

No, I haven't done it. The reason is, as far as I know, if I block an specific@hotmail.com address, all@hotmail.com addresses will be affected, or I'm I wrong?

It shouldn't, unless you want to ban "@hotmail.com"

No, I want to ban just one hotmail account. So, if I ban that one hotmail account, it won't affect other hotmail accounts?
I'm asking this because of something that happens with the banned word "ambien". In Spanish, "También" means "also", but you can't comment using "también" because the spam list asumes it is "ambien". Won't the same thing happen with hotmails if I block just one hotmail account?

OK, I blocked the specific email address and it seems all other hotmail accounts are not affected...

First of all, you will follow the Safe and blocked senders link under Junk e-mail.Now click Blocked senders.Type the undesired email address under Blocked e-mail address or domain.You can block all addresses at a domain — the domain being what comes after the '@' sign in an email address — by entering just that.