It really depends on the level of the ban, and how much effort one decides to go to enforce it. Can someone with a workable knowledge of a proxy get around an IP ban? absolutly. But only if they were smart enough to never have posted with their origional IP address in the first place. There are regionally in any given are of the country only 3-5 options for internet connectivity. Once those are also burned, you tend to be out of luck aside from posting from public internet places like oh, for example the chicago university library (/cough godraiden cough)
The amusing thing is we can ban accounts faster than someone can create them, and one doesn't necessarily have to even ever post on an account for us to be able to remove it. We have certain tools available that were built solely for the purpose of identifying known troublemaker accounts and killing them before they can post. Obviously things slip through there, but its pretty effective for the most part. Which is really hard to prove "Hey, look at all the forum spam you didn't get!", etc.
The biggest waste of time any spammer has ever attempted, was someone wrote a script to create thousands of random yahoo.com e-mail addresses, create thousands of accounts here to go with them (that loophole has since been fixed) and then tried to set up a mass PM spamming campaign. unfortunatly for them, they didn't know about the PM spam limiters, so what was likely dozens of hours of hard work, only ever ended up spamming maybe 20-30 people, and I got to ban thousands of accounts, which is actually still pretty fun.