»Tsst« was really very clever. Take into account that by that point the writers admitted Cartman had become a kind of sub-human monster. They couldn’t top him any more, so they took him back down.
The Super-Nannies treated him as a normal child, but of course Cartman has become like a hybrid of Adolf Hitler and Skeletor in the later episodes (the early ones, he was just a bratty fat kid who used whining and begging to make his mom do what he wants, not unlike how most kids are these days…). Treating him as a child didn’t work as he saw it as insulting his intelligence. Cartman knows that a lot of people don’t like him and he doesn’t particuarly care most of the time. Being friendly is a sign of weakness, punishments are just another way of saying he’s done something that bothers you.
The answer? Bring in a dog trainer who gives him a taste of what it’s like to be treated as less than human. If you refuse to act like a human being, you can’t expect to be treated like one.