public OnPlayerSpawn(playerid)
{
    if(Player[playerid][Banned] == 1)
    {
        Kick(playerid)
        SendClientMessage(playerid,-1,"You are banned have a good life :P.");
    }
    return 1;
}

public OnPlayerConnect(playerid)
{
    new file[128], pName[MAX_PLAYER_NAME];
    GetPlayerName(playerid, pName, MAX_PLAYER_NAME);
    format(file, sizeof(file), "Users/%s.ini", pName);
    if(!fexist(file)) 
    {
        ShowPlayerDialog(playerid, 1, DIALOG_STYLE_INPUT, "Please Register", "Enter Your Password Below", "Register", "Cancel"); 
    }
    else 
    {
        Player[playerid][Banned] = dini_Int(file, "Banned");
        if(Player[playerid][Banned] == 0)
        {
            ShowPlayerDialog(playerid, 2, DIALOG_STYLE_INPUT, "Please Login", "Enter Your Password Below", "Login", "Cancel");
        }
        else
        {
            SendClientMessage(playerid, RED, "You are banned gtfo");
            Kick(playerid);
         }
    }
    return 1;
}

new file[32],Name[32];//Variable declaration, don't declare it here, just make a new one at the top of the script, this is just for the purpose of helping you.
        GetPlayerName(playerid,Name,sizeof(Name));// Getting the player's name.
        format(file,sizeof(file),"Accounts/%s.ini",Name);//You can see where I have 'Accounts' change that to the name of the folder in which the files are stored.
        PlayerData[playerid][Banned] = dini_Int(file, "Banned");//This is storing what's in the file, into the Player[playerid][Banned] variable.