[Laronic]

pawn Code:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_STATE_PASSENGER)
    {
        SetPlayerArmedWeapon(playerid, 0);
    }
    return true;
}

This is setting the "driveby" weapon to "Unarmed (no guns)" for the passenger of a vehicle

Edit: Too late as usually