[[MG]Dimi]

PHP код:

public OnPlayerRequestClass(playerid, classid)
{
     if(classid == [your class id, you will get it by counting, 1st added class is id 1, second 2 etc.])
     {
          GameTextForPlayer(playerid,"This class isn't finished yet!",3000,6);
          return 0;
     }
     return 1;
} 


EDIT: @Pinguin

PHP код:

public OnPlayerRequestClass(playerid, classid)
{
    if(classid == 5) return SendClientMessage(playerid, 0xFFFFFF, "That skin/class is not allowed!");
    return 1;
} 


Will fail. You don't have code to prevent player from spawning and you added teamid instead of classid.