public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate = PLAYER_STATE_PASSENGER)
    {
        switch(GetPlayerWeapon(playerid))
        {
            case 24: SetPlayerArmedWeapon(playerid, 0);
            //like that basically, using case <weaponid>:
            case 0..24: SetPlayerArmedWeapon(playerid, 0);
            //that's every id from 0 to 24 (example)
        }
    }
    return 1 
}