public OnPlayerStateChange(playerid, newstate, oldstate) 
{
    if(newstate == PLAYER_STATE_PASSENGER) 
    {
        new wep = GetPlayerWeapon(playerid);
        switch(wep) 
        {
            case 24: 
            {
                SetPlayerArmedWeapon(playerid, 0);
            }
        }
    }
}