new locked = 0;
new passerver = 0;


public OnPlayerConnect...
{
     if(locked == 1)
     {
        //Make the dialog!
        return 1;
     }
}


//dialogresponse

if(dialogid == ID_DIALOG)
{
     if(strval(inputtext) == passerver)
     {
      //if is right, do what you wanna.
     }
     else
     {
      SendClientMessage(playerid, 0xFFFFFFAA, "Wrong server password!");
      Kick(playerid);
      return 1;
     }
}


//the command

if(strcmp(cmd,"/lockserver", true) == 0)
    {
        if(IsPlayerConnected(playerid))
        {
           if(locked == 0)
           {
                locked = 1;
                //message...
           }
           else
           {
                locked = 0;
                //message unlocked.
           }
        }
        return 1;
    }
    

if(strcmp(cmd, "/serverpassword", true) == 0)
    {
        tmp = strtok(cmdtext, idx);
        if(!strlen(tmp))
        {
            SendClientMessage(playerid, COLOR_GRAD2, "USE: /serverpassword [password]");
            return 1;
        }
        new passwordx = strval(tmp)
        GetPlayerName(playerid, sendername, sizeof(sendername));
        passerver = passwordx;
        format(string, sizeof(string), "New server password %d.", passwordx);
        SendClientMessage(playerid, 0xFFFFFFAA, string);
        return 1;
    }