This won't work in a @colyseus/proxy so it's not really a good solution.

I want to know if is there a way to do something like the following:

const server = new colyseus.Server({ ... });

// Somewhere on a room:

...
async onJoin(client) {
   if (someLogin) {
        // This disconnects a client based on id
       this.server.disconnect(anotherClientId);
   }
}
...

I would like such a feature, to be able to make a client only join a single room at a time, for example. Or if an account is logged in two computers, disconnect the older connection.

For the moment I have found one can "hack" it using something like:

  const Storage = require("dom-storage");
  global.WebSocket = require("uws");
  global.window = global;
  global.localStorage = new Storage(null, { strict: true });

But it has many problems, like populating the global namespace.
If this is still not good enough, one may prefer to create colyseus.js/server and use that inside node. I don't believe it should be required a new package to make a node integration, mostly because a lot of the code should be the same.