'Monopoly' is a good one too. I don't think the terms are mutually exclusive:
"They want the Government to create a Prohibition so they can have a Monopoly on content"
That said, I do think 'Prohibition' is an accurate description. SOPA may well be the first step in a 'War on Piracy' that does have strong parallels with '20s prohibition and the drug war. SOPA is a futile attempt to stop normal human behavior - sharing of content. It won't work. The fact that it won't work could lead to even more draconian blocking measures, creating a self-reinforcing loop of ever-escalating restrictions and punishments. 'Prohibition' conveys that futility and extremeness pretty well for a single term.
I don't like 'Monopoly' because most people don't recognize that as a fundamental problem. In fact in the case of media, monopoly is assumed as necessary to provide you with what you want (hundred million dollar blockbuster films).
The key thing to focus on is the imbalance of power SOPA creates and how that will end up getting in the way of Uncle Art's evening entertainment. So far the shoplifting shutting down the store analogy is the most pointed I've heard, but still not a catch phrase.
I find the shoplifting analogy decent, but the main flaw I see is that it keeps the discussion framed around crimes being committed and what is the punishment. Even if the punishment is seen as really extreme, centuries of legislation have shown that being "tough on crime" is extremely popular.
My hope with the term 'Prohibition' is to turn it around and put the focus on the evil being done by the media companies. They want the government take away your rights, because they don't want to compete fair and square with web technologies.