15 years?! I didn't know moderators could set arbitrary dates for a "temporary suspension". A month would be reasonable for a first offense I would think.
It was an automatically created "session" user (http://meta.stackoverflow.com/questions/44557/why-should-i-r...), an anonymous user given an ad hoc account which they can later convert into a normal account. I don't know that issuing this ban in itself blocks that person from signing up for another account tomorrow. Places like Wikipedia get more sophisticated to prevent so-called "sock puppetry", I'm not sure if SO does the same automatically when an account is banned.
Accounts are cheap. Free, in fact. Length of suspension should be related to the severity of the offenses (in this case, vandalism and abusive/offensive speech) and the likelihood to commit additional offenses (in this case, high because the user vandalized the question many times).