new string[128];
format(string, sizeof(string), "Admin %s[%i] just has banned player %s[%i]. [Reason: %s]", PlayerName(playerid), playerid, PlayerName(id), id, param2);
SendClientMessageToAll(0xFF0000FF, string); // I will send a message to all at first

// "Flag" the player to be banned with a unix timestamp.
Player[playerid][Ban] = gettime() + 3000; // That will set the variable to be 3 seconds from now.
format(Player[playerid][BanReason], sizeof(Player[playerid][BanReason]), "%s", param2); // Also stor ethe ban reason.

public OnPlayerUpdate(playerid)
{
    if(Player[playerid][Ban] != 0) // Adding this check first so it doesn't have to execute gettime() every check.
    {
        if(Player[playerid][Ban] < gettime()) // if its less than the current time then 3 seconds have passed.
        {
            BanEx(playerid, Player[playerid][BanReason]); // I will ban the player
        }
    }
}