[rant start]
I consider blocking ranges of IPs just because they are dynamic unfair. True, spammers use them. But also much more bona fide people than spammers use dynamic addresses. Condemning us all because there are some rotten fruits in the same bag is as unfair as imprisoning a bunch of people because one of them committed some kind of crime, but the exact culprit can not be determined.
However, it is recommended that dial-up users use their providers relay for exactly this reason. In fact, some providers even block port 25 making it impossible for people on dial-up to run their own mail-server. But I agree that completely blocking based on SORBS or other dial-up lists is not The Right Thing(R) to do.
And the exact culprit can be got, with a court order, in Spain. The providers, by law, keep a list of each connection with the phone number used to connect each time. Spammers can be known. It only needs determination on the part of the law enforcing establishment, politics, police, judges, etc, to get at them.
Start with the provider I would suggest. Telefonica was blacklisted by ahbl.org because they refused to do anything (or at least ignored the requests) about spammers. /Per Jessen, Zürich -- http://www.spamchek.com/freetrial - sign up for your free 30-day trial now!