[SickAttack]

More than one player version (spam might occur with more players with the same ip online, but none of them will be able to log into rcon).

pawn Code:

// [ DEVELOPMENT GAMEMODE ]

// INCLUDES:

#include <a_samp>
#include <foreach>

// MAIN:

main()
{
    print("Development Mode: rcon_protection_multiple.amx");
}

// CALLBACKS:

public OnGameModeInit()
{   
    return 1;
}

public OnGameModeExit()
{
    return 1;
}

public OnRconLoginAttempt(ip[], password[], success)
{
    new playerid[3] = -1, count;
    foreach(new i: Player)
    {
        if(strcmp(ip, GetPlayerIP(i), true) == 0)
        {
            playerid[count] = i;
            count ++;

            if(count == 3) break;
        }
    }

    for(new i = 0; i < 3; i ++)
    {
        if(playerid[i] != -1)
        {
            if(success)
            {
                if(IsPlayerAdmin(playerid[i])) // Change this for your admin system's condition that validates a player's admin level possession.
                {
                    printf("[RCON] %s (%d) has logged-in.", PlayerName(playerid[i]), playerid[i]);
                }
                else
                {
                    printf("[RCON] %s (%d) was kicked from the server (Bad RCON Login).", PlayerName(playerid[i]), playerid[i]);
                    Kick(playerid[i]);
                }
            }
        }
    }
    return 1;
}

// FUNCTIONS:

stock PlayerName(playerid)
{
    new name[MAX_PLAYER_NAME];
    GetPlayerName(playerid, name, MAX_PLAYER_NAME);
    return name;
}

stock GetPlayerIP(playerid)
{
    new ip[16];
    GetPlayerIp(playerid, ip, sizeof(ip));
    return ip;
}

One player version (if two players with the same ip address are online, the first one will be kicked and the second one will log into rcon):

pawn Code:

// [ DEVELOPMENT GAMEMODE ]

// INCLUDES:

#include <a_samp>
#include <foreach>

// MAIN:

main()
{
    print("Development Mode: rcon_protection.amx");
}

// CALLBACKS:

public OnGameModeInit()
{   
    return 1;
}

public OnGameModeExit()
{
    return 1;
}

public OnRconLoginAttempt(ip[], password[], success)
{
    new playerid = -1;
    foreach(new i: Player)
    {
        if(strcmp(ip, GetPlayerIP(i), true) == 0)
        {
            playerid = i;
            break;
        }
    }

    if(playerid != -1)
    {
        if(success)
        {
            if(IsPlayerAdmin(playerid)) // Change this for your admin system's condition that validates a player's admin level possession.
            {
                printf("[RCON] %s (%d) has logged-in.", PlayerName(playerid), playerid);
            }
            else
            {
                printf("[RCON] %s (%d) was kicked from the server (Bad RCON Login).", PlayerName(playerid), playerid);
                Kick(playerid);
            }
        }
    }
    return 1;
}

// FUNCTIONS:

stock PlayerName(playerid)
{
    new name[MAX_PLAYER_NAME];
    GetPlayerName(playerid, name, MAX_PLAYER_NAME);
    return name;
}

stock GetPlayerIP(playerid)
{
    new ip[16];
    GetPlayerIp(playerid, ip, sizeof(ip));
    return ip;
}

The rest is up to you, whether you allow more than one player with the same ip address connect to your server or not.