We dont have access to see what the players are running inside their computer, so it's currently not possible for server to kick them right after they've joined with hack enabled. The only way to detect it is just by comparing the difference between the player with hack and without hack( e.g: the speed, action..etc ), but it could be inaccurate due to lag..etc and the inaccuracy will cause trouble for normal players. It's not that easy to add anti-hack, all servers owner would probably add anti-hack if it's possible, unlesss the server has no rules and its owner himself use hack..