forward WantedRemover(playerid);
public WantedRemover(playerid)
{
    SetPlayerWantedLevel(playerid, GetPlayerWantedLevel(playerid) - 1);
}

new timerWantedRemover[MAX_PLAYERS];

public OnPlayerConnect(playerid)
{
    // Set a timer to call the WantedRemover function every 30 minutes.
    timerWantedRemover[playerid] = SetTimerEx("WantedRemover", 30 * 60000, true, "i", playerid);
}

public OnPlayerDisconnect(playerid, reason)
{
    // Killing the timer because we don't need it when the player is disconnected.
    KillTimer(timerWantedRemover[playerid]);
}

// The function to remove the player's wanted level.
forward WantedRemover(playerid);
public WantedRemover(playerid)
{
    if ( GetPlayerWantedLevel(playerid) > 0 ) // If player has 1 or more stars
    {
        SetPlayerWantedLevel(playerid, GetPlayerWantedLevel(playerid) - 1);
        SendClientMessage(playerid, -1, "A wanted star removed.");
    }
}