[antonio112]

You can either use OnPlayerEnterVehicle or OnPlayerStateChange:

pawn Code:

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
        if(/* is player zombie variable */)
                 ClearAnimations(playerid);
    return 1;
}

or

pawn Code:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_STATE_DRIVER)
    {
                if(/*IsPlayerZombie Variable*/)
                {
                     new Float: Pos[4];
                     GetPlayerPos(playerid, Pos[0], Pos[1], Pos[2]);
                     SetPlayerPos(playerid, Pos[0], Pos[1] + 2, Pos[2]);
                }
    }
    return 1;
}