Do you believe that the Nazi party is no different from any of those other parties, or is this just a cheap rhetorical device? Be honest.
The difference between the Nazis and many other regimes is in degree, not type. Where is the line drawn?
Exactly.
I would suggest simply forbidding political userboxes, removing them from any userpage on which they appear and unbanning this user.
Remember, there is a difference between modern neo-nazi parties and the German Nazi party of the 1930s and 40s. They may have the same political beliefs, but they are different parties. This user is not a mass murderer, he just happens to agree with one on political issues.
And I don't think Godwin's law applies to a discussion about Nazis. We aren't comparing anything to Nazis, in fact, we're comparing Nazis to other things - perhaps a converse to Godwin's law? ;)