The spawn of an anarchy server is usually characterized by intense destruction, endless cobblestone layers, and a lacking of any useful resources, including trees, food, or even dirt. It is usually the most challenging part of surviving an anarchy server, and it takes many tries to get through. Despite this, it is by no means impossible to get out. The best way to restrict newbies from escaping spawn is to extend the destruction seen at spawn, but other methods have been tried. Here's why they fail. Note that we are assuming that the methods are perfectly done (no gaps) and no one outside spawn tries to help by tearing it down.
The first thing that comes to mind is the lava wall. If someone builds a giant wall of lava around spawn, no one will be able to get through right? Well, no. First of all, the lava wall doesn't even extend to bedrock most of the time, meaning that all someone has to do is go under the wall. Secondly, even for the ones that do go all the way to bedrock, it is just way to easy to destroy it: I mean, one water bucket can pretty much finish it.
First of all, to even create the wall requires a massive amount of obsidian. Second of all, breaking obisdian with hands is possible. So all that work can simply be reversed in several minutes.
Another option is to simply flood the spawn with water. This looks promising until you realize that the newbies can just swim away from spawn, which is easier than traversing through the cobblestone mountains. There are also some ice biomes near spawn, so you've just basically made them a free pathway out.