new g_Ignored[MAX_PLAYERS] = {INVALID_PLAYER_ID, ...};

CMD:ignore(playerid, params[])
{
    if(isnull(params)) return 0;
    new id = strval(params);
    g_Ignored[playerid] = id;
    return 1;
}

CMD:pm(playerid, params[])
{
    if(isnull(params)) return 0;
    new id = strval(params);

    if(g_Ignored[id] == playerid) return SendClientMessage(playerid, -1, "That user is ignoring you!");
    return 1;
}