While blocking IPs is far from a perfect solution, it is more often effective than you might realize - and I learned this from keeping close track of my site logs. Also, it is possible to distinguish between static and dynamic IPs in very many cases.

Blocking emails is fairly effective; the liklihood that the email account will be deleted by the provider, then another person will try to sign up at this site using the same email address in the future is vanishingly small. Not zero, of course, but small.

The basic problem is that there is no perfect (or even nearly perfect) solution for spam. The best we can do these days is make our sites hard targets, requiring validation and manual effort by the spammer, because that spam which takes time is spam that won't make money. Only the most dedicated and determined spammer will keep going back to a site which promptly deleted his username (which he had to create manually), blocked his email account (which he had to set up manually) and blocks his preferred IP (which causes him to do more work to relay or proxy).

I can kill the IDs, and block the emails and the IPs for less time and effort than the spammer invests in setting them up. I still spend too much time on it, so lately I have taken to blocking entire nations from my message boards if I have a spam problem from them. I know, I know. But they're not my customers anyway and I don't care if their access to my site is restricted...particularly when practically all the traffic I get from their nation is trouble for me.

The point is that the measures I propose are not perfect, but they are better than the alternatives.